4koma (pronounced yonkoma) is a Japanese word meaning "4-panel"1, and refers to a type of manga which is formatted as a series of four vertically-stacked panels. This term is used in contrast with "regular" (non-4koma) manga, which do not typically adhere to a rigid panel structure.2
Here is an example from one very long-running (and famous in Japan) 4koma manga, Sazae-san:3
Some of the most famous 4koma manga among Western audiences are Azumanga Daioh, Lucky Star, Hetalia, and K-ON. As this list suggests, the 4koma format is heavily populated by gag comedy manga.
1 from 四 yon "four" + 齣 koma "scene, frame, panel"; usually written 四コマ today.
2 Think of 4koma as being sort of like your bog-standard weekday newspaper cartoons (at least in the US) in that sense - 3 or 4 panels, and no real freedom as to how to use the space available.
3 Note that most 4koma these days have narrower panels (usually square), allowing two 4koma to be printed side-by-side on paper with a standard A or B aspect ratio