Since it hasn't been properly explained so far...

There are two differences 'twixt' beer/ale & lager. The first is top/bottom brewing. Lager is top brewed - the yeast sits at the top of the must (must: the fermentable liquid), beer/ale is bottom brewed. Nothing to do with the must, AFAIK, it's just the way the yeast is.

Difference two: Lager is supposed to be drunk cold, beer *not*!. This is one of the reasons I hate lager. Ever druk warm lager? It's horrible!

The ideal serving temperature for lager is @4C, beer should be served at 'room' temperature, 12-16C. This explains my standard commentary on lager ("You know why they serve it that cold? It's so you can't taste it!"). Being slightly more fair, lager is supposed to be drunk cold, it's brewed with that end in mind.

Beer you should be able to taste. Comments have been made about the temperature at which Guinness is served. It is Irish stout; it is beer, not lager; you should be able to taste it! The notion of ice cold Guinness, to me at least, is bloody ridiculous. Whether that alters any perceptions of Guinness, I don't know; but as someone who likes his beer, I'm always happy to try anything that claims to be a stout. I'm not overmuch on bitter, but mild, stout, & plain simple 'beer'...

Not so happy about anything that claims to be "Irish" stout, though. I've always found them to be... lacking... (this includes Guinness, but I've never drunk in Ireland, so I may have been unlucky).