File association is the way that Windows knows what program to be opened by a file. .doc files normally open Microsoft Word. This is done by associating certain file extensions (the last part after the period in a file name that is usually 3 or 4 characters.
File associations also is how Windows decides to show certain icons by certain file names in Windows explorer or on the desktop. For instance .pdf files after associated with Adobe Acrobat and have a fancy A icon (icons are the pictures on the desktop and beside file names in Windows Explorer.
File associations are normally set by a program when you install the program. Occasionally file associations are pointing to different program than you want. .htm or .html files are the most common pointing to wrong program. By default they point to Internet Explorer, but if you install Chrome usually they are changed to point to Chrome. If you use Chrome regular that is probably what you want but if you really want to use Internet Explorer normally this is a problem as anytime you click on a .htm file you open Chrome.
You can change your file associations in Windows.
In Windows 7 to change file associations go to Start, Default Programs, Associate a file type or Program. Then search for the file type (the extension), and choose it and then click Change program button. Then you will choose the program you want associated with it.
You can also set the file associations all back to the default (original) settings by choosing Set program access and computer defaults at the Default Programs menu