Do PIC micorcontrollers NEED an external oscillator?
Yes, the 16F84 does need an external oscillator. It is a very old PIC. 

However almost all of the newer PICs have an Internal RC Oscillator that can be selected, which will be mentioned in the datasheet.  

I would really consider getting hold of a newer PIC, something like a 16F690, or 16F1824/16F1828. These are far more current, and can do anything the 16F84 can do and much more.

If you want to use your 16F84 though, either use an external clock (e.g. from 555 timer or oscillator based on e.g. an inverting gate with RC or crystal) or crystal as specified in the datasheet, or if you don't have an external clock or crystal use the RC option.