I believe that beer is a general term used for lager, as in "Do you fancy a beer", where mostly it infers a pint / drink of any kind, namely lager, although a lager is not a beer.
I think that beer is actually ale; real ales being the malty flat ones like Morlands Original, Speckled Hen, Bishops Finger, IPA etc that are stored in a keg at room temp, and lagers being the fizzy ones like Stella, Fosters, Kronenbourg, etc.
Hope this link helps:
http://en.wikipedia.org/wiki/Beer#Styles