An irregular galaxy is one that does not have an obvious shape like a spiral or elliptical galaxy. In other words, irregular galaxies do not fall into any of the identifiable classes of the Hubble sequence.
Irregular galaxies are often chaotic in appearance and no show trace of spiral structure or a central bulge. Quite often, the chaotic shape results from a recent merger or interaction between nearby galaxies.
Also known as peculiar galaxies, these non-conformists are thought to make up around a quarter of all known galaxies. In general, irregular galaxies contain large amounts of gas and dust, and can exhibit a significant degree of star formation activity.
The closest galaxy to the Milky Way happens to be an irregular galaxy called the Canis Major Dwarf. With just one billion stars, it appears to be a small galaxy in the process of being consumed by our own galaxy.