What makes a assault rifle different than other rifles?

Depends on whether you're referring to an actual assault rifle, or a so-called "civilian assault weapon" as defined by the BATFE.

In the case of a true assault rifle, they're defined by their use of an intermediate cartridge combined with the select fire option (burst or full auto). Larger rifles which fire full power cartridges, such as the FN FAL or G3, fall into the battle rifle category.

For semi automatic, civilian legal firearms of the sort there's a concerted effort to ban currently in the US, the BATFE definition (and the basis of the 1994 Clinton ban) considers a rifle to be a so-called "civilian assault weapon" if it has two or more of the following features:

• Pronounced pistol grip
• Flash suppressor or a threaded barrel
• Collapsible or folding buttstock
• Bayonet lug
• Rifle grenade launching lug

California law defines one as having even a single one of those features (except for the bayonet lug), and also includes thumbhole stocks and forward grips.