Laboratory reports are required any time a scientist (and science student) conducts a laboratory activity or does primary research (that is, where you get your own data without looking it up anywhere). Two types of primary research are possible in science: experimental and descriptive studies.
In an experiment, you formulate a hypothesis (i.e., an educated guess) about an observation you've made in nature. The hypothesis gives you a basis for testing your observation in a controlled manner. If it is a good hypothesis, the results will lead you to either accept or reject the hypothesis as stated.
In a descriptive study, you collect data but are not testing a hypothesis. For example, if you dissect a frog, you are collecting qualitative data on what you observed during the dissection. The qualitative data might consist of both written descriptions and diagrams.
In both types of laboratory activities, your formal lab report should follow the same basic format that follows.