Just as a note: There is no such thing as a Roth 401k. The term "401k" is simply a tax code to the IRS, but has become synonymous with a retirement account. You're thinking of a Roth IRA, which stands for Individual Retirement Account.
actually a roth 401k is exactly what one would assume. after tax retirement contributions offered by one's employer. it's a relatively new compared to traditional 401k so not many workplaces offer it. the contribution limit for roth 401k is same as traditional, but effectively higher since it's post-tax vs pre-tax.
The Pension Protection Act of 2006, signed into law by President Bush on 8/17/06, made the Roth 401(k) permanent, removing the 12/31/10 expiration date that previously was in force. (8/17/06).