when u save files without any extensions ... ie like in front page u click save as webpage it will be saved as .htm file ... it does not pic l for some reason ... rest i dont know
Older operating systems only allowed three characters in the extension. Nowadays you don't have enough fingers to count. But people sometimes use 3 byte abbreviations for compatibility. So html is abbreviated to htm, and that's why.
Nothing from an output point of view. They're both HTML files.
However, (filename).html and (filename).htm will be interpreted as two different files. This may lead to duplicate content issues with search engines (among other issues) down the road. Stick with one (my choice is .htm, but as long as you're consistent it doesn't matter.)
