There is no difference besides the extra letter. Way back in the day when systems could only read 3 letter extentions, htm was all that was possible. the name convention allowed html which seems more appropriate.
Servers search for index pages they are set up to search for. if you list .htm, .html, .php, .shtml it will look for these pages. There is no difference in performance, they are parsed the same.
If youre looking for a "better" extention, use html. ppl are more used to that