Their bread and butter was, and as far as I know still is, trading in ETFs and ADRs. These things have a mechanism to create and redeem in exchange with underlying securities. This means one can make some money arbitraging differences between the bundle and components.

As this game got more competitive they seem to have branched out into other kinds of business including HFT.

The page says their hardware team builds the fastest trading systems on the planet. Jane Street does HFT.