what is the SPD ram setting

well i can set my ram to either 100/133/or SPD , i have the jumper set to 133, and set to 133 in BIOS, then i just up the FSb to whereever i want it. What will it do if the jumper is set to 133 on the board but i choose SPD in BIOS, what is SPD exactly

Its what the memory mfg. programs the spd of the memory module to read by the bios.
If you buy memory rated cas 3 by the mfg. then they program the memory so the bios will set the memory timing to cas 3.

Decent quality DIMM's have a little chip on them that reports the important info about the Ram to your mainboard, as Placid posted. "By SPD" works like he said, too. SPD stands for "Serial Presence Detect." The mobo queries the DIMM's for this info. In the absence of you selecting CAS, etc in BIOS, your BIOS runs the DIMM's "By (the) SPD."

so why do they have the cas setting in the same block as the mem settings, its

100
133
SPD , and only 1 is chooseable at a timne