afterglow82 wrote:Bus - from what I read, this seems to be a kind of heavy hitting, probably fast moving unit with good protection, aiming to get into combat quickly?
For "Bus" you also need to cross reference "Death Star".
"Death Star" refers to a heavy hitting close combat infantry/monstrous infantry unit packed with combat characters. A "Bus" is the cavalry/monstrous cavalry equivalent of a "Death Star"
For comp, what you'll generally come across are changes organisers make to the way you can build your army in an attempt to achieve theorectical balance between the armies. There are lots of different comp systems, as people have mentioned, that get used at a number of events but some tournament organisers roll their own depending on the metagame in their area.
As regards "uncomped" it means no changes to the rules or scenarios that affect how you build your army (which is why it's bit rich that some areas boast their tournaments are "uncomped" Warhammer, by which they mean they don't impose any hard or soft unit restrictions, but then use non-rulebook scenarios or add in "objectives" or other in-game tasks that do force you to tailor your list to them if you want to have any hope of doing well).
ETC comp tends to be a bit controversial as it's generally considered quite hard comp and also changes core rules (e.g. restricting the number of magic dice you can use to cast spells from the lores of Death and Shadow, a different Line of Sight system). It achieved special notoriety for the first ETC using the 8th edition rulebook as the first few revisions of the comp basically looked like those writing it wanted everyone to go back to playing 7th edition.
Needless to say, most people have their own opinion on how much comp is too much.
