To be honest joden, it's kind of a case where if you have to ask that question in the first place, you probably won't have much need just yet. As for differences...
Boost vs concrete - boost doesn't sound very good. You can limit softly and it might sound clean, but try to go medium or heavy limiting and it just sounds pumpy and distorted. Concrete limiter just limits without these bad artifacts unless you push it much harder. It will limit your audio much more than you can with boost.
PC2A is just a different sounding compressor. It's very smooth and reacts differently to different material sources in a nice way. It's based of the LA2A. Look it up for an idea of what sound it gives. If you don't have a good grasp on compressors, it probably won't be a lot of use to you, though it is very easy to use, so in that case could be better for a beginner.
You get the idea about the other two. Like the PC2A, they just give a different sound for the more experienced engineer to work with.