Chain Kombos
Kombo Attacks are a sequence of Normal Moves that can be strung together with the proper button sequence. Every character has several Kombo Attacks, and each character's Kombo Attacks are unique. Knowing your Kombo Attacks just requires memorization.
Kombo Attacks will get you more damage than Normal Moves alone. They are also very useful in Juggle Combos, as many Kombo Attacks are designed so that all hits juggle against airborne opponents very conveniently.
Kombo Attack Leniency
The window for performing Kombo Attacks is pretty lenient when it comes to timing. In fact, there is almost no pernalty for doing Kombo Attacks too quickly. For example, Scorpion has a Kombo Attack of Forward + Back Punch into Front Punch into Back Kick. You can literally hold Forward on the controller and press all three buttons in the sequence before the first attack even connects, and the entire chain combo will execute.
It's more likely that you'll perform the Kombo Attack too slowly. Generally speaking, if the attack has connected and you hit the button a little after that, the next move in the sequence won't come out. So if you are going to perform the Kombo Attack, you're better off inputting the commands quickly. However, this does make for difficulties when it comes to hit confirmation as you do not want to commit to a Kombo Attack that leaves you vulnerable if the opponent blocks it, so you may have to perform it a little slowly so you can stop before the last hit if the opponent is blocking.