What Exactly Happens When I Clear App Data?

[Serengeti Lion]

Hi, Since I've had weeks of trouble when searching Google, trying to watch a video on YouTube, I finally got tired of it and called Google. They advised me to do a couple of things with one being to clear the data in my apps that I have trouble with. I know about clearing the cache and I did that weeks ago and it didn't really help. What I want to know is, what EXACTLY happens when one clears the data in an app? Having asked that, I am aware doing this takes care of slow issues. I am having the problem of Google giving me search results totally irrelevant to what I asked for and the other day it took 35 minutes to watch 12 minutes of a video because the connection to YouTube's server kept crashing. I don't have an SD card to back up to but do have the cloud storage. I don't want to lose videos I have liked under personal categories or my subscriptions or things I have on my Google+ page, so what will clearing the data clear when I do it? Thanks