They are very similar in meaning.　Recipe directions will usually say 'simmer for 1 hour', rather than 'stew for 1 hour'. And yes, simmering is generally done on the cook top rather than the oven. I think 'stew' is most often used these days as a noun - a beef stew. So you could say, I simmered this beef stew for nearly 2 hours. Actually, I just remembered, I do still see 'stew' used as a verb, but most often with fruit - 'stew the apple until soft'. I should add that I speak British English - Americans may use different terminology.

For me (American), stew refers to either the type of dish (beef stew), or the pot in which I would cook stews (stew pot). A stew pot is used to cook stews, dishes that are slow cooked over several hours to achieve a thick sauce. In other words, you would simmer a dish over a long period of time to to turn it into a stew. I would never refer to this process as stewing, but I suppose you could. I would still understand you.

I would say they are very similar, or maybe the same, in terms of cooking process. The only difference I can think of is that STEWED can be added to another food word to create the name of a dish: Would you like some STEWED tomatoes? SIMMERED is not generally used this way.