Very basically, a clade is a grouping of organisms ( that currently exist or have existed) with a particular common traits. 

Wiki says...

A clade (from Ancient Greek κλάδος, klados, "branch") or monophylum (see monophyletic) is a group consisting of an ancestor and all its descendants, a single "branch" on the "tree of life". 

The ancestor may be an individual, a population or even a species (extinct or extant). Many familiar groups, rodents and insects for example, are clades; others, like lizards and monkeys, are not (lizards excludes snakes, monkeys excludes apes and humans).

Cladogram (family tree) of a biological group (ancestor at the bottom). The red and blue boxes at right and left represent clades (i.e., complete branches). The green box in the middle is not a clade, but rather represents an evolutionary grade, an incomplete group, because the blue clade at left is descended from it, but is excluded. image: wikipedia

Increasingly, taxonomists try to avoid naming taxa that are not clades.