New Tristram at night.

New Tristram is one of the towns in Diablo III. It's a wretched, rotting corpse of a town, grown up on the ruins of the old Tristram that blossomed during past ages, but has now fallen into decay.

WWI 2008: D3 Lore and Environmental Art Panel Q&A session revealed that New Tristram is the starting town of Diablo III.

Q: Will we see how the cities have changed in the last 20 years?

A: I can tell you that the game starts in new Tristram.

Tristram can be seen in the official world map.
Numerous screenshots of battles in the fog-shrouded ruins of Tristram have been released.

Inside Tristram Cathedral.

Lore

There is a New Tristram lore entry in the official page. The following quotation summarizes the rise and fall of New Tristram:

"New Tristram has been in existence for several years, though the exact date of its founding is unclear. Originally simply a collection of merchants looking to profit on adventurers and travelers drawn by legends of riches within the old cathedral, it slowly set down roots and became an established town. As soon as the cathedral was looted bare, however, the adventurers and travelers stopped coming, and New Tristram found itself in decline. The town is now comprised mostly of depressing shacks; the inn is the only building that looks even the least bit habitable."