SPOT foreign exchange is exchange of two cash currencies in two days from time of trade apart from Canadian dollar which is one day. Spot foreign exchange is traded on the interbank market by means of over the counter trading between the two banks or through an inter-dealer broker, however, using leverage retail investors can participate in the interbank market but it is not really worth it yet as it is still in its early times. You can find quotes for spot prices in the financial times, on bloomberg website, on most bank websites.
Forwards are a derivative but are also traded on the interbank market they are cash transactions again like spot but are for a specified time in the future eg USD/EUR 3 month forward.
Currency futures are different, they are an exchange traded future and are a contract trade unlike a forward which is a cash trade. Currency options are also traded on a derivatives exchange. In terms of liquidity spot fx most liquid market in world for developed world currencies ie USD/EUR USD/GBP USD/JPY, then there are the illiquid currencies of the emerging markets like Thai Baht, Mexican Peso, South African Rand these are not so liquid. The derivatives markets in the case of futures and options are fairly illiquid and the forwards are highly liquid.
My advice to you is to read 'Trading the Global Currency Markets' you will glean a lot of information from there. Hope this helps.