Yes, a simulater lets you test certain fragments of code on your pc, to see it acts like you would like it to. But there is a limit to simulating, not all perhepials can be simulated.
Picbasic pro compiles your basic code into pic assembler before an acutal hex file is produced. So you'll always be able to simulate the produced assembly with any pic simulator. But i've never seen a simulator for pic-basic wich simulates on the basic language level itself.
A bootloader allows certain pics to self-program. The bootloader program is first put into the pic using normal programming methods (a normal programmer). Then you can use the bootloader to get a new program from any source the pic is capable of interfacing with (a pc connected to the serial port for example) and let the pic write it to its own program memory.
Not all pics are capable of this , as far as i know only the 16F87X series and most flash 18F's can