[*]Growth: Good possiblity for high profits and thus high returns. However, that is already priced into the stock, meaning it costs you a lot of money up front for this (you get less stock per dollar you invest). And of course if the high growth isn't there, that stock plummets.
[*]Value: Consistant returns, profits are probably consistant. It isn't fancy, so the price is low. You get more stock for your money. But likely it won't skyrocket either direction.
[*]Blend: Basically in the middle of the two.
[*]Small cap: small companies that either go belly up or make it big. Can be a good money maker but can also be more risky.
[*]Large cap: big, usually sound companies. Priced already at a premium.
[*]Mid Cap: Basically in the middle of the two.
You basically want each. You want diversification. If one stock gains a lot - you have it and you gain a lot. If one of your stock falls, you only have a little and it doesn't hurt you much because you didn't put all your eggs in one basket. Diversification for the win! Thus, you want large value companies; you want large growth companies; you want small value companies; and just a little bit of small growth companies (use less of this since historically returns are average but risk is high).
Any good mix of funds like that will cover your mid-cap and blend companies, so you can forget about them.
Now repeat all of this internationally too, but at half the level of your domestic stocks.
Then add 10%-20% bonds to even things out (bonds and stocks often go in opposite directions, so a little bit of bonds helps lower volitility drastically without impacting returns much at all).
A pretty good mix would thus look like this:
[*]1/8th large growth - domestic stocks.
[*]1/8th large value - domestic stocks.
[*]1/8th small value - domestic stocks.
[*]1/16th small growth - domestic stocks.
[*]1/16th large growth - foreign stocks.
[*]1/16th large value - foreign stocks.
[*]1/16th small value - foreign stocks.
[*]1/32th small growth - foreign stocks.
[*]20% bonds.
[*]The remaining 15% in several specialized areas, just a few percentage of each: metals, REITs, etc.
Of course these are approximate levels, and you won't do bad even if you go so far as to double one of these or eliminate another. And if you are just starting, you probably don't have enough money to actually buy that many items. But, it is just a good goal to have and something to eventually strive for.