Mountain bike suspension fork
if your tires say '26 × 1.79' the you have '26 inch' wheels also know as ISO/ETRTO 599, i.e. the rim diameter is 599mm. the '1.79' is the nominal width of the tire in inches. 

Mountain bikes all used to have this size wheel until '29 inch' appeared (ISO/ETRTO 622) and then the compromise '27.5 inch' (ISO/ETRTO 584) - hence the options you see on the website.

To fit your frame you need to make sure the crown to axle length of the new fork is the same as you current fork, otherwise you will raise or lower your headtube and change your head tube angle. If you are buying a fork with the same travel then I'm guessing the fork length will be about the same.