if your going to create web apps, you need to know ajax, its the way forward for interactive rich apps, jquery is a javascript library(the most poplaur one i think), and it has built in ajax function. jquery makes it incredbily easy to deal ajax. so yeh defo learn jquery, its important and mad easy!!

EDIT: sorry i forgot to mention what ajax is, ajax is a way to communicate with your server without having to relaod the page, getting instant feedback i.e. like commenting on facebook.