Hebrew is the language that the Old Testament was written in, although it was later translated into Greek and Latin before being rendered into the currently more-familiar English translations. The Hebrew Bible was not yet fully canonized when it was first translated into Greek, and as a result a few of the texts of the Old Testament, as found in Christian bibles, differ from the Hebrew Bible in use by Jews. The fact that it is the source language of so much of the Bible is what gives Hebrew its religious and historical importance, as many of the arguments concerning the Bible eventually refer back to the Hebrew original.
The Bible was written over hundreds of years and contains oral traditions that are even more ancient, and its language accordingly changes somewhat from part to part. Modern Hebrew is likewise not Biblical Hebrew, having amassed even more changes and lost much of its Biblical vocabulary and even some of its grammar. But these differences are relatively small, and for the most part a modern Israeli, conversant with Modern Hebrew, can pick up the Hebrew Bible and read it (with the exceptions of a few sections written in Aramaic, and an occasional confusion where Biblical Hebrew deviates from the Modern one). Indeed, the most substantial change to occur over all these millenia was simply the transition into the modern Aramaic script; ancient Hebrew was written down in a different "font", although in the same alphabet.
In comparison with English, Hebrew is noteworthy in that its alphabet contains no vowels, only consonants. While later (medieval) versions of the Bible added a system of symbols to elucidate pronunciation, the core and ancient language had none - which often left words ambiguous and open to different interpretations. For example, the word הלך (he-lamed-kaf) can be read as "Halach" meaning "went" or "Helech" meaning "traveler"; such ambiguity can lead to very different interpretations of some verses. This difficulty is compounded by the fact that ancient Hebrew also didn't include marks for separating words (spaces in modern English and Hebrew), and the Hebrew Bible originally had no division into Chapters or paragraphs (such divisions were added much later).
The Hebrew Language: A Brief Overview
The Hebrew language is based around three-consonant "roots", that are put into particular patterns to convey different senses in which the core-idea is used. For example, the root "הלך" (he-lamed-kaf) signifies the idea of "walking". "Halach" (saying the first and second consonants of the root with the sound "a" as in "Land") means "(he) walked" or figuratively "he went", whereas "Halcha" means "(she) went", "Holechet" means "she is walking", and so on. The various ways a word can be changed are fairly regular, so for example the root "מעד" (Mem-Ain-Daled, stumble) can be used to say "Ma'ad" (he stumbled), "Ma'ada" (she stumbled), "Moedet" (she is stumbling), and so on. Often suffixes and prefixes are added to the root to make these changes. For example, the prefix "Ha" means "The", so "HaHelech" (ההלך) means "The Traveler".
There are a few exceptions, including exceptional words and inclinations as well as shorter or longer roots, but the core of the language lies in this way of modifying the root.
Biblical Hebrew used an alphabet of 22 consonants. A few letters were probably pronounced differently based on their surrounding consonants and there were likely some exceptions, so the number of consonants used may very well have been higher. Although these were not noted down, Biblical Hebrew also had as many as 12 vowels (some words also contained consonants that were uttered without a vowel, like "pan" in English). Modern Hebrew retains only 5 vowels and while it retains the ancient alphabet a few consonants have merged and are now pronounced identically; however, it also added a few compound-letters using the apostrophe (') to introduce foreign sounds (for example, the "J" sound in Giraffe ג'ירפה).
There is no point to listing the entire Hebrew alphabet and vowels here. For this, see Wikipedia:Biblical Hebrew and Wikipedia:Hebrew Alphabet. Instead, we only note that in general the first consonant of the letter's name is how it's pronounced. So, for example, the letter Gimel is a hard G as in Galipoli.
For English-speaking people, Hebrew consonants usually sound hard and its speech stochastic, but Biblical Hebrew was more tonal and included longer, continuing sounds. The stress was usually on the penultimate syllable (i.e. like U-ni-ver-sal rather than Ul-ti-mate-ly).
Sentence structure is flexible, but in modern Hebrew most commonly follows the form Subject-Verb-Noun. So for example just as an English speaker will say "Moses went to Egypt", a Hebrew speaker would say "Moshe [Moses] Halach [Went] LeMitzraim [To Egypt]". Biblical Hebrew often uses a "reversed" word order of Verb-Subject-Noun, so the same sentence might be written "VaYelech [And Went] Moshe [Moses] Mitzraiyma [To Egypt, in a more archaic form]".
All nouns in Hebrew are either male or female. There is no neutral, "it", gender. So for example a table is a "he", the sun is a "she", and so on. If you run into an Israeli saying something like "This table - he is beautiful", that is because in his native tongue (and hence in his mind) the table is a "he", not an "it".
Modern Hebrew has only three tenses - past, present, and future. Biblical Hebrew, however, had a bundle, including various "perfect" tenses.
Another point to note that while English is written down from left to right Hebrew is written down from right to left. So the letter sequence Aleph-Lamed אל has in English the Aleph (א) left to the Lamed (ל) while in Hebrew it is to the right. The same is true for words - the sequence El Elyon אל עליון has Elyon (עליון) to the right of El (אל) in English, but to the left in Hebrew.
To see all of this in action, let us consider the first sentence in the Hebrew Bible,
בראשית ברא אלוהים את השמים ואת הארץ
Usually translated as
In the beginning God created the heaven and the earth
The first word uses the root Resh-Aleph-Shin ראש, which marks the idea of head. By extension, it also serves as to mark the leader, as in "head of state", and therefore can also serve at times as the "first" or "beginning". Thus one arrives from the root Resh-Aleph-Shin ראש to the word Reshit ראשית, which is of a structure that marks objects and means an initial point or beginning. Then one adds the prefix be ב, which means inside or in. Thus BeReshit בראשית means "In the beginning".
The next word is based on the root ברא, meaning "create". It is usually pronounced Bara ברא, which puts it into a past-tense verb form, thus "Created". This is not the only possible interpretation, however. Other considerations lead to the idea that it should be pronounced Bro ברא (written identically in the original ancient Hebrew, since there were no symbols for vowels!), which is a perfect-passive-past form indicating something like "while the creation was going on". Thus the two words may have originally meant In the beginning [of God's] creation...]".
The third word is derived from the root word El אל, meaning god. The word Eloha אלוה derived from it is a later word meaning god as well, and the plural suffix Im ים is added to signify plurality. Thus Elohin אלוהים means "may gods". In modern Biblical readings, however, and in modern and probably also late-Biblical Hebrew, the word Elohim אלוהים was simply considered another name for God, the singular deity.
Thus the first sentence of the text may just as well have originally read "In the beginning of the gods' creation of the heaven and the earth". But it can also mean what the typical translation says - "In the beginning God created the heaven and the earth". Both are valid translations.
We hope this demonstrates the difficulties in interpreting the Hebrew Bible and reading its text. Obviously tradition, context, contrast with other parts of the text or other contemporary religions, literary analysis and the documentary hypothesis, and so on can all assist the interpretation and understanding of the text. Different interpretations need not be given equal weight and plausibility, and not all interpretations and readings are valid. Still, reading the ancient Biblical texts that are the source for the Old Testament and providing a good English translation is not an easy task, and this is why to this day new translations, versions, and editions are produced.