Observation is the act of acquiring information based on one or more of the senses. For example, visual information is gained through sight, auditory information through hearing, and so forth. An observation is one piece of such information. A set of observations may be called data.
Note that observations may come through intermediary technologies. For example, we can't see radio waves directly, but we can build electronic components called radio receivers that respond in predicable ways to such waves. The responses of the electronic components may then be observed through sight or hearing.
A set of observations is different from an explanation of those observations. In science, for example, a hypothesis may be developed as a possible explanation of one set of observations and tested based on other, independent sets of observations. When a hypothesis successfully explains many such independently observed data, it may become a scientific theory.