Photography...i just bought a casio exilim 7.2 mp camera just wanna ask something?

do i need to always max the resolution of my camera in 7.2mega pixels in taking pictures?..and whats the difference between digital cameras and SLR?..i know price has a big difference but i wonder why 10mega pixels digital cameras are cheaper compared to SLR cameras which only has 4mega pixels..hmm was it the lens of SLR or what not?..i just wanna know bout it:)