Welcome to the madhouse Jane.
I started reading the books very early on in the series and enjoyed the laugh out loud humour. Over the years, I have grown to appreciate the more satirical and philosophical side to his work.
He also has the ability to satirise taboo subjects (such as sexism, racism, religion, etc) without causing offense. In fact, when I first read TCOM and TLF, I was surprised that I not only found his references to HP Lovecraft's works to be very funny, but that I didn't have to take my literature quite so seriously! (I would have fitted in well with the 'hang anyone who criticizes LoTR' brigade in those days).
Mind you, I think you'll find a diverse range of answers when the others get around to this thread. Terry seems to offer different things to different people.
The difference between fiction and reality? Fiction has to make sense.