Originally Posted by BBear 
for the color gels, I have the vivitar color kit, which has all the basic colors and fit right in the flash slot. really nice.
now for color temperature, I have the Rosco Gels cut to fit my vivitar flashhead... and they dont look nice as the vivitar original color filters, but they do the work!
Please excuse my ignorance. I'm very new to flash photography (I just ordered a Pentax AF-360 and am wondering what kind of accessories to get).
What is the difference between a color gel in the flash slot and a color temperature gel to cover the flash head? I can understand the latter is to modify the color temperature of the light coming from the flash, but what does the former do?
As for diffusers: are they really necessary? Do you only need a diffuser if you are taking a picture with a wider lens than the flash range covers, or do diffusers have other uses?