Originally posted by OXCOM developers:

OpenXcom is an open-source clone of the original UFO: Enemy Unknown (X-Com: UFO Defense in USA), licensed under the GPL and written in C++ / SDL. It was originally founded by SupSuper in February 2009, and has since grown into a small development team surrounded by a very supporting community.

The goal of the project is to bring back the tried and true feel of the original with none of the issues. All the same graphics, sound and gameplay with a brand new codebase written from scratch. This should give it:

Fixability: Play the game natively without any need for emulators or fancy hacks, with none of the limitations and bugs that plagued the original. No more 80-item-limit, personnel limits, funding overflows, disconnected facilities, broken proximity grenades, floating soldiers, etc.
Moddability: Tweak the game to your heart’s content. Sure the original was pretty good, but maybe you just think it could be that bit better. A nicer base layout, better laser weapons, maybe challenge yourself with a custom game mode, or just put in all the crazy stuff you’ve always wanted! None of it is hardcoded.
Flexibility: Port the game to any platform you like, customize it to your liking, or use it to make your own far-fetched remakes. The code is fully documented and open-sourc so anyone can take a crack at it.

[...]steam\steamapps\common\xcom ufo defense\XCOM