The parts of a paragraph

Boiled down, a paragraph is composed of a topic sentence and several secondary sentences. The topic sentence can appear anywhere in the paragraph, though often it's the first sentence. It states the paragraph's main idea. The secondary sentences support the topic sentence by providing details, direct evidence, illustrations, or personal observations.

Here's an example, from a student paper on geography:

Vegetation in the Mono Basin varies from the Mono Lake shoreline to the upland areas of the basin. Near the lake, we find alkali-tolerant perennial herbs. These hardy plants thrive here, having adapted to the soil's lack of moisture as well as its alkalinity. As we move farther from the lake, however, desert scrub-zone vegetation, such as sage and rabbitbrush, begins to dominate the landscape.

The structure of this paragraph is conventional (which is, of course, just fine). The first sentence is the topic sentence: it tells us that the paragraph is about vegetation in the Mono Lake Basin. The rest of the sentences provide specific information about the topic.