People pack in lunches. Disney doesn't have a "no outside food" rule in their parks.
They will not let you carry in alcohol. And, of course, things like guns, knives, fireworks are not permitted. You can't bring anything in that pulls behind you (a rolling cooler, a wagon for the kids). You can't bring in wheels (like rollerblades or skateboards - Heelies are against the rules but the rule is seldom enforced). Pets are not permitted (of course, service animals are).
You cannot wear a Universal Studios shirt. ;-)