Extensions

Writer
Application Document - .sdw
Master Document - .sgl

Calc
.sdc

Impress
.sdd

Draw
.sda

Base
.sdb

Math
.smf

Writer: a decent word processor. Some people say it's better than MS Word.
Calc: the OpenOffice spreadsheet program and a replacement for MS Excel. 
Impress: a presentation program that replaces MS PowerPoint. 
Draw: a two- and three-dimensional drawing program that many people rate more highly than the same functions within MS Office.