Basically Tomb Raider 1 but with a few add-ons, notably climbable walls, shallow water, ziplines and outdoor levels with visible sky. You can drive boats in the Venice level and skidoos/snowmobiles in the Tibet level.
From Wikipedia :
The majority commented that while the differences from the original game are relatively small, Tomb Raider was a strong enough game that these differences and the consistent execution were enough to make an outstanding game.