The main difference between a 401 K and a Roth IRA is when and how the taxes are paid on them.
With a 401 K you make contributions to the 401 K fund from Pre Tax dollars and you get a tax break upfront which can lower your current income tax bill.
With a Roth IRA it's more tax efficient though and the Roth IRA is tax free when it comes time to withdraw money from the Roth IRA.
With a Roth IRA the amount of money you can contribute is less than what you can put into a 401 K fund.