Actually, I think this is a great question. So many Melee players have come to me telling me that there are absolutely no combos in Brawl.
I won't pretend to be an expert. Brawl is my first fighting game and I started it around the time of APEX 2012. But I love this game, so I have done a decent amount of research.
According to
http://www.ssbwiki.com/Combo, a combo is a series of attacks that are guaranteed or very likely to hit once the first attack hits. I want to differentiate between combo, where the series of attacks are very likely to hit, and follow-up, where the series of attacks are more uncoordinated and based on the reaction or lack thereof of the opponent.
A true combo would be inescapable. Like the posters before me said, chaingrabs are true combos in that once you hit the first attack, which is a grab, and if performed correctly, they are IMPOSSIBLE to get out of. Grab release combos are considered grab release for a reason as well. Marth's grab release combo on Meta Knight will hit Meta Knight no matter what he does, provided the Marth executed properly.
Normal Combos, like InfiniteV115 has accurately stated above, are percent dependent and are subject to break by SDI. Snake's Ftlit, a natural combo consisting of two strikes, can be escaped using SDI, and professional players do it all the time. Falco's Gatling Combo only works on low percents. However, for all intents and purposes, these are considered combos.
I don't have much experience playing Melee, but I am almost sure that any of their defined combos, such as their Ken Combos, are also subject to percent dependency, although at a much less noticable scale. Melee has little guaranteed combos as well, as almost all of their defined combos are also subject to escape by good DI and SDI (see Fox uthrow to uair on Jigglypuff).
But that is the point of Brawl. I like Brawl because of its gameplay revolving around followups. I like Brawl because it focuses more on option coverage than tech skill. So to answer your question, Brawl has a little of every type of combo, but don't focus on it too much, as it doesn't revolve around these combos.
Hope this helped
