I have no idea what IM and Trillian are, but I find Skype to be rather lightweight and very convenient for most of my needs as far as keeping in touch with people is concerned. You can just have your status on 'busy' if you don't want to receive calls.
Hey, if you like it and it works for you that is good, keep with it. As for me, I can't put myself in busy on my mobile but I keep invisible there, unless Skype randomly changes me to online. I've had people trying to call me like at 4 am. 
That said putting myself in busy also means I don't get sound sor nnotifications so even if people tries to text me I'll probably won't notice in time.
IM = Instant Message or Instant messenger.
Any software that allows you exchange messages quickly in real time can be called an IM.
For example AIM (Aol's IM and YIM, Yahoo's)
Trillian is a cross-protocol Messenger application that allows you to sign in to different accounts at the same time. So through Trillian I am online in facebook Messenger, YIM, AIM and other IM protocols. Trillian is old and it looks outdated and to be honest I installed it rather recently. So don't worry too much about it.