There are two types of knockdown: "soft" or normal knockdown, and hard knockdown.
In the usual knockdown state, the default action is to get up normally after a second or two (known as wake up). Players can also quick stand upon knockdown by pressing buttons to recover faster.
If an opponent is stunned, the knockdown resulting from that attack becomes hard knockdown by default.
Hard knockdown can be used to create unavoidable setups; in Ultra Street Fighter IV, the Delayed Wake-Up mechanic was introduced as a remedy, allowing players to slow their recovery and throw off foes.