The most important difference between a Roth IRA and a 401k is that Roth IRA contributions are made with POST-tax money, and 401k contributions are typically made with PRE-tax money (taken directly from your paycheck, before taxes are withheld).
Another important difference is the contribution limit. A Roth IRA, in 2011, allows you to contribute $5,000 if you are under age 50, $6,000 if you are over 50, with some income limitations (high income earners are not eligible). A 401k allows you to contribute up to $16,500 in 2011.
Distributions are also taxed differently. Assuming you are of retirement age (age 59 1/2), Roth IRA distributions are tax-free (since you paid taxes BEFORE you contributed). A 401k is typically "rolled" into an IRA, at which point distributions are taxed as ordinary income.
While these are some general differences, these are not the only differences. Check out my hub Roth IRA 101 for some additional info regarding Roth IRA basics: