All this data is potentially out of date, and should be taken with a truckload of salt
Isk is the main currency used by capsuleers within the Eve Online universe.
1 isk is the equivelant to 1 credit (dollar, pound, euro)
Without this currency, making any headway in the Eve Universe is a long and sometimes "Impossible" task.
Isk can be sourced via:
and various other sources within the Eve Universe
Isk is used for many things from buying ships and commodities to paying for corporation rent, deposits on courier contracts & upgrading clones.
Please note: this is just a short list and not at all complete