Internal frame backpacks are exactly what they sound like: they have a frame that is built into the interior of the fabric of the backpack. There is no way to detach the backpack from the frame, and you cannot affix anything to the bars. The benefits of an internal frame pack are that they are typically a little more compact and distribute weight evenly with a narrower design. External frame backpacks have a frame that is on the outside, and the backpack hooks to the frame itself. One downside to external frames is that they are a bit more cumbersome and make it more difficult to navigate through small spaces or rocky terrain.