A definition paragraph is often part of first paragraph, the introductory paragraph, of an essay. It is an explanation of terms in the essay, or the topic of the essay. This can be accomplished through the use of synonyms, explaining which class the topic falls, and an explanation of what the topic is not.
With respect to "The Birds," a definition paragraph can, for example, explicate the du Maurier narrative as a gothic tale [See --http://www.virtualsalt.com/gothic.htm] The thesis, then, that comes at the end of the introductory paragraph will be a general statement of how these gothic elements presents work toward the development of the plot or of the theme.
A descriptive paragraph is a paragraph that creates for the reader a sensory image of whatever is being described. It is important to be detailed and create a single impression with this paragraph. Thus, in writing a descriptive paragraph as part of an essay on "The Birds," the student may wish to use this paragraph as support for the thesis that du Maurier's tale is gothic. For instance, in an explanation of the horror and danger, the descriptive paragraph can point to the psychological horror of birds as destroyers, detailing incidents from the narrative, such as the following:
They were not intent upon their flight, as the crows, as the jackdaws had been. They still circled overhead. Nor did they fly so high. It was as though they waited upon some signal. As Though some decision had yet to be given. The order was not clear.
In this paragraph, the oddity of the birds' behavior as well as the suspense of what they will do indicates the unnaturalness of their behavior, the beginning of the psychological horror.