A "host" is the nervcentre of a Digital Audio Workstation (DAW). Here, you access all your virtual instruments and effects, record your audio and/or MIDI data tracks, and hopefully get it all playing nice together in real time without crashing the computer...

Cockos REAPER

Cockos REAPER -- cheap, stable and immensely powerful. Also, has the most useful and fair-play "try-before-you-buy" license scheme in the history of audio software!

Top Ten Software Cantabile

For live purposes, or when you just want to chain some synths and effects together to program some sounds or try something out, Cantabile is the way to go. Loads almost instantly, extremely stable, and doesn't get in your way.