The tutorial of Robi Pilikar brings technical details about it:
http://users.rowan.edu/~polikar/WAVELETS/WTpart1.html
But, in few words:
" the wavelet is a tool that permit to analyse the variability of the signal in several scales (cm, meters, km ...) and times (seconds, min, hours, days, years, ...), all together in the same time"
