Neither of the above posts is fully correct on the definitions.
Backdoor Roth IRA = non-deductible contribution to a traditional IRA, immediately convert to Roth IRA. This gets you around Roth IRA income limits.
Mega Backdoor Roth = after-tax (NOT ROTH - there are 3 types of contributions you can make to a 401K) contributions to a traditional 401K, and rollover to Roth IRA as soon as possible. This works best with in-service withdrawals also part of the 401K, but can be a net positive even if you have to wait until you leave to do the rollover. This one is basically an almost ridiculous thing for those with 401Ks that allow it - not getting around any specific limit - just using the tax code / IRS interpretation of it to its fullest extent.
stashing_it is right on the Backdoor Roth IRA process.