i-TTL is a fancy way for saying “automatic flash”. In other words, you set your flash to i-TTL, and just fire away; it automatically adjusts the power to what is needed to properly expose the subject. It’s just like your on-board flash, you don’t have to think about it, or adjust it, it just works. Here’s an article that might throw some light on the subject. (Pardon the pun:)
Manual flash on the other hand has to be set manually. You set the power level, and the flash will always fire at that power.
CLS is Nikon’s Creative Lighting System. Here’s a video tutorial on how it works. (Be prepared to turn down the volume, it starts out rather loudly with some “noise”.) I also found a PDF which seems to explain more about it. It seems that there is very little information on i-TTL and CLS.
As mentioned elsewhere David Hobby’s Lighting 101 and Lighting 102 are the last stop for learning how to use external lighting. (Unfortunately, “there is not a sense of moral integrity in the packaging in which the content is presented” to quote @JamesStaddon.)
I want to make sure that I get somthing that will last, and preform very well, as the job I’ll be doing has to be done right the first time!
If you can get the SB-700 used, then it will probably be your better option if that’s what you’re looking for. Even so, you can still get 3 of the Yongnuo’s for the same price. But as @timtam mentioned, you do get a case diffuser and two color correction snap on filters with the SB-700.
I have used two speedlights for lighting up a room for a group portrait of 50-60 people. Even then, it was just barely enough light! I wish I had done more experimenting now, because I think the placement of the lights has a lot to do with it.