The term prosody refers to the study of versification.  Most prosody begins with an analysis of metre. Metre (or meter) is derived from the Greek word for "measure."  The metre is the pattern of stressed and unstressed syllables in  verse.  While there may be some variation or substitution, the number of syllables, and the number of stressed/unstressed syllables remains relatively consistent from line to line. 

The most common metres include:

A metrical unit of a line is called a foot.  A foot consists of one stressed syllable and one or more unstressed syllables.  Each type of foot is denoted by a specific term (line breaks are indicated by "/"):

Meter is usually described as either the dominant foot (which foot is used most often to the strongest effect in a work),  Or the dominant number of feet per line.   Generally, though, critics combine the dominant foot and number of feet to describe meter.  That is were common terms like iambic pentameter and trochaic tetrameter gain their critical meaning.