but note that when targeting pages, this.htm is not the same as this.html, always good to adopt one naming convention, and to show your files extensions from the folder options if you are a microsoft user
AFAIK the .htm is an invention of M$ so the .html will fit their three letter file extensions:
some examples:
file.exe
file.txt
file.htm
I usually use .html as I think that it is the proper extension of a html page. not that it matters much as both ways will work fine.
Just makes sure that your server goes for the extension you specify. I don't think all servers will check both for index.htm and index.html for example. But I could be wrong I guess.