what is a 'trainer'
 

when you get upadates / mods / patches i have noticed that some of them are called 'trainers' is this just a different name for the same thing?

I'm not 100 % sure but almost :) it's not the same thing, it's like a small cheating application you run before you start the game it's meant for, you don't shut it down when you play, then you can press certain keys (or combinations) hopefully to be seen in the trainer itself, to activate somthing in the game, like if you press 'E' all enemies becomes blind :) or if you press CTRL+F you get all weapons, you get the point I guess...

btw I'm from Sweden so I reserve myself for any eventual spelling error :)

A trainer is someone who teaches you something ie. a fitness trainer will teach you how to lose weight and tone up :D a soccer trainer will teach you how to play soccer :D or another definition would be that a trainer is a type of shoe many people in america refer to them as sneakers. hope this help you:D

Dell Dimensional is closest :)

It's a program that runs in the background that changes memory variables in the game which in turn allow you to cheat.

So you launch the trainer program, then load your game, then alt+tab back to the trainer to activate the cheats (if the trainer has buttons to click) or do what DD said and press a key in game that the trainer has set to a cheat.