The majority of cars use the OBDII system or the equivalent European (thank the EU!) EOBD,only the very latest cars have a different system (CANBUS).
A basic Fault Code Reader (FCR) will read codes from the OBDII / EOBD systems.
The difference in price of the FCR reflects what it does,the very basic £25.00 ones will generally just display the codes and you will then have to look it up in the booklet provided,more up market ones will show the code and its meaning.
Again dependent on price some will allow you to reset service lights and the EML after sorting the fault but some functions for ABS systems etc. require a specialised dealer tool for example Vauxhalls Tech 2 system.
So it all depends on what you want it to do and how much you want to spend.
Car Mechanics magazine recommendations this month go for the Autel MaxiScan MS409 priced around £63.00,this gives all code details,stored codes,freeze frame and live data, check www.talktomycar.co.uk for details