My understanding is that companies will match your IRA contribution up to a certain percentage, oftentimes having the investment initially be in company stock. After a certain length of time, the money is put into a IRA and, potentially, funds of your choosing.
I tend to favor independent IRAs over 401k's, as I am not sure I will be making a career out of my current job, and while the money can be moved around if I were to leave, it seems like an unnecessary hassle that can be avoided by investing independently.
If you do pursue an independent IRA, I would certainly recommend a Roth, as it is nice getting the taxes out of the way initially, so that when you withdraw as a senior citizen, you will be able to withdraw the money without taxes being applied to the "grown" amount.
As for the details of why a Roth is better, I just know it is ideal. I tried explaining it to a friend through mathematics, but I'm an English major and math is not my strong field.
