system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is a 401k?
gpt
A 401k is a special savings account with three rules:

1) You pay much less tax on what goes into a 401k (and, if you want, don't have to pay any tax at all on it until you take the money out)

2) You can only put up to $18,000/year into the account

3) You can't take the money out until you turn 59.5 years old (you actually can, but you pay a penalty that totally removes the value in putting the money there in the first place)

The program was created by the US federal government to incentive people to save for retirement. Typically you pay a percentage of each paycheck into your 401k, and most employers will match what you put in up to a certain percentage (usually 2-6% of your paycheck). Employers do this as a benefit; whatever percent they match can effectively be seen as a raise, which is pretty nice.