Though It's a somewhat difficult thing to understand in the teachings (with quite a bit of debate)...
There is the Jewish concept of "Sheol", where the dead go. "Hell" is often referred to in the King James Version of the bible since it is used as a translation for "Sheol" and "Hades". Thus seeing the word hell does not necessarily refer to the interpretation of the Lake of Fire we are familiar with referring to hell as.
While not widely and popularly discussed, it is held by some Christians that the classic "lake of fire" Hell will not be populated until the final judgement and that those unbelievers awaiting judgment wait in Sheol for that day (Thus they are doomed to go there, but are not there yet). Believers are believed to immediately be accepted into heaven however through Jesus, as evidenced by his statement "today you will be with me in paradise" to a man next to him on the cross being crucified with him.
This is supported in the bible in Revelation 20:13 and 14.
Rev 20:13: "The sea gave up the dead who were in it, and Death and Hades delivered up the dead who were in them. And they were judged, each one according to their works."
Note that Hades appears actually as "Hades" here, it refers to Sheol (which is just the Hebrew word for it). This implies that people are waiting in Hades (the Jewish Sheol) for judgment and will be judged at the time of judgment in the end times.
Rev 20:14 "Then Death and Hades were cast into the lake of fire. This is the second death."
So after the people who are in Sheol/Hades are judged and found to lack in salvation, they are then cast into the lake of fire. Note this is an event in the future.
Food for thought.