An Ulcer is simply the destruction of the skins surface tissues which results in an open sore. There are several types depending on where they are situated.
You mention legs and feet so I am hoping you are talking about a Venous Ulcer which occurs on the lower leg or ankle. Usually caused by chronic hypertension in the deep leg veins, due to the destruction of the valvular system in the vein. This is usually first seen as an oedema (swelling), sometimes eczema and bleeding into the skin that leaves a brown stain.
Here is a link to a Video with a talk by a Consultant about the subject: For some reason you can only copy and paste this one ?
nhs.uk/video/pages/medialibrary.aspx?Id={DDDC1AC3-E7FC-4B58-A71A-BAC707930137}&Uri=video/2010/2.February/Pages/Venouslegulcer.aspx
IF ANYBODY IS SQUEAMISH.......PLEASE DO NOT WATCH THE VIDEO.
Ken