I asked the same question way back, in a slightly different manner - I wanted to open a Roth IRA and didn't know where to start. What I gathered after some research was if you are a young buck, you can take digs at some aggresive but risky funds, which swing wildly, but you have all the time in the world to play hit and trial, make some buck, and get the f.u.c.k. out of there before it is too late to a more stable fund. If you are already in your 40s or even 35+ play safe as you are past your wild streak. A co-worker of mine is starting Roth IRA at 23 and he swings like pendulum between different funds and making lot of dough. I can only imagine he will have saved a substantial amount of money by the time he retires.
I am sure there are lot of experienced folks, money managers or folks who work in finance in sajha. their advice would be so valuable for the uninitiated.