Q:

What wine did people drink during the Roman empire?

What was the wine like that people drank during the times of the Roman empire? How alcoholic was it? I heard they diluted their wine with water, but why would they do that?

A:

You have a lot of information right out of Wikipedia when searching for Ancient Rome + wine, and too much for it to be copied or gathered here. But here is some information I also learnt from History courses or readings.
Basically, many wines (Greece, France, Italy...) were kind of "sour" and people were used to adding spices (among other things), not only to "sweeten" the taste, but also to help store the wine and keep it drinkable for a longer time. (1)
Some of the wines the Romans used to drink (there may be more than that, of course):

Mulsum -> wine with spices and honey (served during the gustatio, before the meal)
Turriculae -> dry wine, with sea-water, fenugreek and defrutum (2).
Carenum ->  sweet and liquorous, from very mature / ripe grapes, with quince, various plants and defrutum.
Vinum picatum -> (pitched wine is the closest I can come with in English) wine that was sweetened by the pitch used to seal the container.

Why would they add many different plants / herbs / honey / water?

Ref. Wikipedia : Romans drank their wine mixed with water, or in "mixed drinks" with flavorings. Mulsum was a mulled sweet wine, and apsinthium was a wormwood-flavored forerunner of absinthe. Although wine was enjoyed regularly, and the Augustan poet Horace coined the expression "truth in wine" (in vino veritas), drunkenness was disparaged. It was a Roman stereotype that Gauls had an excessive love of wine, and drinking wine "straight" (purum or merum = unmixed) was a mark of the "barbarian".
It seems that it was done for 3 purposes:

keep wine longer and help it come from countries hundreds of miles and days of travel away.
make the wine more "drinkable" and enjoyable.
don't do like barbarians do and look like a well-educated person.

-> How alcoholic was it? is a question I have no knowledge of and no answer to.

1 - FWIW: adding spices to a beverage was done again centuries later, when people started drinking hot chocolate (before the refining process / conche was discovered). The taste was so bitter / sour that people put a lot of spices and sugar in it.
2 - A reduction of must in Ancient Roman cuisine, made by boiling down grape juice or must in large kettles until reduced to half of the original volume. See also grape syrup.

Useful links: 1. wines in ancient Rome - 2. History.SE - 3. wine and Rome (University of Chicago) - 4. about ancient roman wines - 5. Rome and wines

