When our focus is on definition, we commonly define a word by using it in the subject position (An X is...), often supply a superordinate, and supply a picture if possible. The following definition of donkey from the Collins Cobuild New Student's Dictionary contains all these features:
"A donkey is an animal like a small horse with long ears => see picture on page 815."
When we focus on collocation, we are more interested in exemplifications, both for analysis and production, as illustrated by the following sentence:
"My donkey helps me carry water four times a day."
Definitions are rather formal affairs. An exemplification, on the other hand, is an example of the word in use, may embody almost any thought, is conversational, and more revealing in terms of a word's collocates.