And you've come to the right place for that extended learning! 
Total market funds try to mimic the performance of all stocks (in this case, US based) by adding some mid and small cap stocks to the core s&p 500. Usually they have about 6000-8000 stocks, cap weighted.
The upshot is there isnt a lot of mid and small cap, they're still usually 70-80%+ large cap s&p500ish funds. The addition of the smaller company stocks may boost returns...but they might not...depends on how the markets move in the future.
The "extended market" fund is pretty much the slice of "total stock market" that isnt the S&P500 piece...ie the mid and small cap hunk. By blending the s&p500 fund and the extended market fund, you can mimic a total market fund but "up" the mid and small cap pieces...which tend to be more volatile and higher returning over time...perhaps quite appropriate for someone your age.
If you want to get fancy and start screwing up your portfolio right off the bat...a split three ways between the s&p 500 fund (cheapest you can find), the extended market fund, and the international index fund wouldnt hurt.
__________________
Many an optimist has become rich by buying out a pessimist
|