DVB-T is OTA SD television content branded as "Freeview". You get over a 100 channels but, to be honest, only about 30 of them are any good. There are all the major stations (BBC 1 and 2, ITV, Channel 4 and 5), their additional channels (BBC 3, ITV 2 etc), some +1 hour channels and some Freeview only channels. Whilst these are all subscription free, there is a small amount of subscription content and it's not essential to subscribe to these. You don't get many of the Sky channels.
DVB-T2 is the same as T but with the inclusion of 10 or so (I can't remember the exact number) HD channels. It's branded "Freeview HD". Again, subscription free for the majority of the channels. It's nice to watch Top Gear in HD.
DVB-S is the same as T2 but, I think, has a few more HD channels. It's branded "Freesat" and requires the installation of a satellite dish on the side of the house - which often fails the WAF test. It arrived before Freeview HD and so was the first way to get HD channels, although I'm not sure whether that really is the case any more.
For those that are wondering, "YouView" is actually a STB with a DVB-T2 tuner and a range of additional catch-up and VOD services bolted on.
The majority of people will probably get DVB-T2.