http://beeradvocate.com/beer/101/ has a vast amount of information on beer styles.
To answer your specific question, an ale is top-fermenting whereas a lager is bottom-fermenting. Lagers are generally fermented at lower temperatures than ales.
I believe that lambics would constitute a third category, since they are traditionally fermented by wild yeasts, but opinion may vary on this, I don't know.
There are dozens of styles of ales and lagers and a few different lambics, but the method of fermentation is the main distinguishing characteristic between the three types.
The similarities…
Fat, flour, cheese, milk
The differences…
Welsh Rarebit - English mustard, Worcestershire sauce
Beer cheese dip - garlic, paprika, dijon mustard, beer [optional in rarebit]
You might swap one for the other, but warn your diners first which they're getting.
If I got garlic in Welsh rarebit I'd probably not eat it, wondering what the heck was wrong with it. if I knew it was meant to be there beforehand, I probably would.
Similar could be said for the Dijon [too vinegary] & paprika [odd colour even before tasting].
I'd eat either & probably enjoy it, so long as I knew what I was getting before I tasted it; otherwise you get that odd mental trip-up like when you think you've been served tea but it's actually coffee. It's revolting until you realise, then it's fine.
BTW, rarebit works really well for Philly cheesesteak ;)
Best Answer
Cans have a lot of advantages over bottles: they don't allow light in (light spoils beer), are easier to stack (and take up less space when arranged tightly), the materials for each can are cheaper, and so on. From the consumer perspective, this results in cheaper and higher-quality beer, all else being equal.
The reason craft breweries have been using glass bottles rather than cans is largely historical and aesthetic. Some hip craft breweries started using cans recently, and it led to a trend.