When following a simple loop, you know you will go on a single trip that takes you back to where you started. There's one journey, and one destination. Strange loops are a bit more complicated — they form a kind of set of instructions, an ordered hierarchy, that brings you higher and higher, until you're back at the beginning where you started.
Strange Loops were the brainchild of Douglas Hofstadter, a philosopher and scientist who wrote I Am a Strange Loop. They can be simple or complex, but they depend on what Hofstadter called "tangled hierarchies." Instead of a linear progression, these hierarchies balance on each other. Together they encompass a set of instructions that set out two equally valid ways of looking at a situation. The situation cannot be resolved without elevating one view and one part of the set of instructions over the other, but there is no objective way to do that.