My brother died @ age 49 and named me as a beneficiaries on his 401k plan thru his employer,?

, I contacted the administrator "Fidelity" and they advised that my options would be to roll this into an "inhertited IRA" or take the cash (20% going to the federal government), I am 46 what is an inherited IRA and do I have to wait til 59 1/2 to draw and how is this different than a 'roth ira". Help!!!!