Cloud mining 101

So, I'm considering getting casually back into mining after a long time (BTC was $5 per when I stopped). I'm well aware that my hardware is totally insufficient for mining and I was looking at cloud mining as an alternative. I'm not looking to be really profitable, I just want to have some BTC change for if I shop at a place that takes it. A lot of the fountains have banners for Cex.io and I was interested. I do have a few questions about it before I jump in feet-first and I looked around and didn't find many answers.

A: How are the GH/s you buy used? Do they have their own pool and you just sign up in it like normal? Do you direct the the GH/s to the desired pool?
B: How are the contracts handled? For example, Cex.io wants 0.0702 BTC per Gh/s, Butterfly Labs wants $10.83 per. Butterfly Labs says that's a 12 month contract. Is that $10 per day? Per month? For the whole 12 month contract?
C: Is that rate of X Gh/s a (nearly) constant rate or is that a flat amount? (i.e. It mines continuously at X GH/s or it mines up to a total of X GH/s)
D: What's a good reputable group? Since I don't have any real BTC at the moment I'd prefer using USD to pay for it.