A hub is like a device to split the wires up to more ports, much like those things you can get for your portable music player so that two people can listen with headphones at the same time, etc.
The problem with this is packet collision: if both of you try and use the line at the same time, collision of data can occur, and while the protocols are made to tolerate this, it can slow your data transfer rate down.
A switch solves this, by having fast electronic switching to alleviate the problem. A switch is what's in a router too (and a router typically has more detailed configurations -- router tables, etc.).
Having said all that, while a switch or router would really be better, for just a few devices, a hub should be fine. If there is any differences in your gameplay, it should really only be a slightly higher ping rate, if at all.