Young earth is like the literal Biblical 7 days. There are lots of theories that stem from this idea. Like a conversation I had recently was about young vs. old. I had never heard the theory he presented, neither had anyone else I talked to... I actually got laughed at for bringing it to the table with my uncle and his buddies... the have numerous credentials between them.
Old Earth is that the "days" talked about it the Bible are not 24 hour days. So in that case a day was millions of years, or thousands or billions depends on the science you study and draw conclusions from. This one has tons of theories that stem form this one as well. It finds its way into the scientific and the religious arenas.
Both are very interesting studies actually. Both use what I see as applied science. Where its convenient for the theory then its applicable other then that science is OUT.
Keep in mind all theories wether secular or religious depend on faith. You must believe in that something was simply there at the beginning. It would be fun to use a thread to study these too... unless you want to debate we can do that too.... Just tell me what side you want me to take