Tacos, burritos, chilli con carne...have a look at Chiquito's menu on their website for a vague idea (although they have a lot of random crap there too, and IMO it's not particularly nice so don't get your hopes up too much if that's where you're going haha). Ooh look at Wahaca, much nicer than Chiquito's.
Chiquito's serve almost a 'Britishfied', cliched menu of Mexican food, i.e cheesy nachos, chilli etc and the place is decorated with sombreros etc. It's a decent enough place to eat out but not really the best representation of Mexican food.
Wahaca as mentioned above is a great, creative chain in London started up by Masterchef winner Thomasina Myers, would deffo recommend that to anyone wanting to try Mexican food.
Expect rice and various breads (wraps and flat-type breads as opposed to yeast based ones, lots of pork and steak, avocado based dishes, flavoursome tomato, chilli spice. Mexican food at it's best it deeelicious!