The retirement account you get through a job — and the closest thing to free money you'll ever be offered.
A 401(k) is a retirement account you get through an employer (public schools and governments have cousins called the 403(b) and 457(b) — same idea). Money comes straight out of your paycheck before you even see it, which makes saving automatic — the ultimate "pay yourself first."
Like an IRA, it holds investments (usually funds) and grows over time. The difference is where it comes from — your job — and one powerful perk most IRAs don't have.
Many employers offer an employer match — they add their own money to your account when you contribute. A common one: "we match 50% of what you put in, up to 6% of your pay."
That is a 50%+ instant return before your investments do anything. Not contributing enough to get the full match is literally leaving free money on the table. (One catch: vesting — you sometimes have to stay at the job a while before the matched money is fully yours to keep.)
Assume your employer matches 50% of the first 6% you contribute. Drag to see your money, their money, and the 30-year payoff.
Whatever else you do, contribute at least enough to get the full employer match — it's the highest-guaranteed "return" in investing. Many people do exactly that in their 401(k), then send extra savings to a Roth IRA for its tax-free growth. You don't have to choose forever; you just start.