Attacking village - A player (You) is sending his/her troops to that city to attack/pillage it.

Onslaught - Multiple players (from your clan, including who or whoever started the onslaught) send troops to that one city (player's city or place of power) to attack it. The number of troops you can send on an onslaught is determined by the level of your War Bloc.

Attacking village is effective only when you go in par or overwhelm the player's city's troops. (ONLY ONE PLAYER)

Onslaught is effective when attacking more powerful enemy players, and this is usually carried out by clans or single players from that clan to join in on the attack. It is pretty much the same with just attacking, just on a larger scale with people.

Hope this made sense 

Keep in mind the STRONGEST HERO needs to lead the Onslaught.  Also, pay attention to what attack he specializes and and send the troops that match that to get minimal losses.