A junior is much like an apprentice. Often they are being told what to do and told (or hopefully TAUGHT) how to do it. They don't show a lot of autonomy. A lot

A junior is much like an apprentice. Often they are being told what to do and told (or hopefully TAUGHT) how to do it. They don't show a lot of autonomy. A lot of mistakes happen from a lack of understanding and foresight. A mid-level developer is mostly autonomous within the confines of a planned out project. Given a specific scope and criteria, they mostly get the job done with some help from a senior. Their errors and bugs are typically of the architectural and performance type, though a lack of foresight can plague the less savvy mid-level. A senior should be nearly completely autonomous. You tell them the general thing that needs doing and they are able to do it on their own. They can see the impact a decision would have far enough down the road to accurately weigh pros and cons. On top of that, they are able to help the mid-levels and juniors get better at their craft as well.