The two words are often used interchangeably, which is wrong.
If a system is complex, it means it has many components in the system. The complexity makes it hard to apply any hard and fast rules for problem solving. Think of a large company or organisation.
If a system is complicated it can be hard to solve, but they are addressable with rules and recipes. Think of a machine.
My next guest on the podcast, Dan Rogatschnig, did a masters degree specialising in this stuff and he laid it out for us during our chat.
Come and have a listen on Friday.