GIMP (GNU Image Manipulation Program) is an full-featured and open source graphics program. In other words, it’s a free alternative to Photoshop.
I’ve used GIMP for years; in fact many of the images on this website have been either created or modified with GIMP.
There are differences to Photoshop, some things are easier and some things are harder, but on average the quality of the end product is indistinguishable. It has the same ability to use layers, paths, masking, varying levels of opacity, etc.
GIMP also benefits from a huge and active global community who are constantly developing different things and are quite happy to share their discoveries with the world. There are a vast number of tutorials and tips for accomplishing effects with GIMP, from the simple to the absolutely spectacular.
If you’re looking for an alternative to Photoshop (especially now that Adobe has moved to a strictly subscription-based cloud model), I would highly recommend that you check out GIMP. I’m sure you’ll feel right at home using it in no time.