The crevice between toes acts as a lint trap. Fibers from socks and shoes, dirt, dead skin cells - they accumulate there, creating what is commonly referred to as toe jam.
However, what lurks between your toes might also be infectious - bacteria and fungi are the common culprits. The fungus may be better seen as a yellowed toe nail (or those creepy green fungus globs from the fungicide commercials...just kidding, you're not likely to have green, moldy looking fungus on your feet!)
Or it could be scabies - parasitic mites that like to live between dirty toes.