Brewing with hops was imported from Flanders in the 14thC. Prior to that hops were not used in England as they were believed to promote melancholy (before you get smart, go & look up the medieval theory of humours).

It's not a strict definition, but if you think of ale as non-hopped & beer as hopped, many people, particularly re-enactors, will understand. This is distinct from the term Real Ale (which I hate!), & also distinct from the technical & biological difference between an ale yeast & a lager yeast.

Ere hops, many things were used to bitter beer, but none of them had the preservative qualities of hops. The two that immediately spring to mind are ground ivy (NOT poison!) & alecost (Tanacetum balsamita, IIRC). The latter I have in my garden. It smells very strongly of mint, but imparts a lemony flavour to your brew. No brew, hopped or othrwise, would taste much like anything modern. If you can find & brew (approximately) to a medieval recipe, though, I recommend you do. They're different. Perhaps not to your taste, but try it & see...