The Catechism of the Catholic Church defines purgatory as a "purification, so as to achieve the holiness necessary to enter the joy of heaven," which is experienced by those "who die in God’s grace and friendship, but still imperfectly purified" (CCC 1030). It notes that "this final purification of the elect . . . is entirely different from the punishment of the damned" (CCC 1031).
The purification is necessary because, as Scripture teaches, nothing unclean will enter the presence of God in heaven (Rev. 21:27) and, while we may die with our mortal sins forgiven, there can still be many impurities in us, specifically venial sins and the temporal punishment due to sins already forgiven. Read More...(Catholic.com)
Also, I found a great Podcast on the topic of purgatory. Many questions you may have can be answered by downloading this podcast. (55 min. program, but still really good, so give yourself some time to listen to this...right click on the link to download and "save target as" to your hard drive)
Hope this helps those who may wonder.