If you max out your 401k contribution ($18,000 if you're under 50) you can also contribute to an IRA (Roth or traditional) but your contribution will not reduce your taxable income.
The two aren't related. If you max out a traditional 401k and your income is low enough that you can take the traditional IRA deduction as well. Though that deduction phases out by $71k/year, which in our field is relatively low.
