The virtual instruments or VSTi's youre talking about such as EastWest QuantumLeap, 8Dio, Symphobia etc etc are sample libraries.
to play them you need a MIDI capable keyboard or other midi input device or by directly playing them from within DAW Digital audio workstation software such as Cubase, Logi Pro etc etc .
From within a DAW you can record midi from your VSTi or you could without a keyboard ener the notes via the softwares piano-roll or the virtual keyboard which Cubase has and probably the others but I only really use Cubase so not 100% sure on that one There are VST keboards available.
Most of these VSTi's libraries use Native Instruments Kontakt as the host which as well as VST andRTAS installations also has has a standalone player version meaning you dont need the DAW software host to play the sounds.
Most of these libraries, the good ones anyway, run at around 8GB upwards, I have a choir one which runs at around 60GB !, the size is due to the multi layered samples (which cheap libraries dont have for all the instruments and cost around £200 plus.
to run these libraries you need a fairly decent PC/MAC with a decent amount of HDD space or as I and lots of others do, put all your sound libraries on a dedicated HDD.
Or you could by one of the all singing dancing keyboards that Yamaha Tyros produce