There is one big difference between a Eurail Pass and an InterRail Pass, and that is Eurail Passes are for sale to non European residents and InterRail Passes are only for sale to European residents.
Then there are some differences in the products offered. For example, the Eurail Global Pass offers travel in 21 countries, whereas the InterRail Global Pass offers travel in 30 countries. The validities, or number of travel days, to choose from is also different for these two passes. Furthermore, InterRail does not offer Regional or Select Passes where Eurail does, but InterRail do offer a larger selection of One Country Passes.
Also to note, the main similarity between Eurail and InterRail Passes is that they both offer excellent freedom and flexibility to explore your choice of European countries by rail!