Each level is different and you can work through them to level up your character, complete more quests, and find more loot. (andymerrill.com)
So while levels are initially linear, you will be revisiting them to find either new powers or quest items to find even more quest items to unlock access to newer levels. (totallygn.com)
New ships have more powerful defense and offensive capabilities, and also can hold more quests so you can earn XP even faster! (thedgcast.com)