ondemand
Available in most kernels, and the default governor in most kernels. When the CPU load reaches a certain point (see "up threshold" in Advanced Settings), ondemand will rapidly scale the CPU up to meet demand, then gradually scale the CPU down when it isn't needed. - SetCPU website

conservative
Available in some kernels. It is similar to the ondemand governor, but will scale the CPU up more gradually to better fit demand. Conservative provides a less responsive experience than ondemand, but can save battery. - SetCPU website

performance
Available in most kernels. It will keep the CPU running at the "max" set value at all times. This is a bit more efficient than simply setting "max" and "min" to the same value and using ondemand because the system will not waste resources scanning for the CPU load. This governor is recommended for stable benchmarking. - SetCPU website

powersave
Available in some kernels. It will keep the CPU running at the "min" set value at all times. - SetCPU website

userspace
A method for controlling the CPU speed that isn't currently used by SetCPU. For best results, do not use the userspace governor. - SetCPU website

Interactive

The 'interactive' governor has a different approach. Instead of sampling the cpu
at a specified rate, the governor will scale the cpu frequency up when coming
out of idle. When the cpu comes out of idle, a timer is configured to fire
within 1-2 ticks. If the cpu is 100% busy from exiting idle to when the timer
fires then we assume the cpu is underpowered and ramp to MAX speed.

If the cpu was not 100% busy, then the governor evaluates the cpu load over the
last 'min_sample_rate' (default 50000 uS) to determine the cpu speed to ramp down

SMARTASS GOVERNOR - is based on the concept of the interactive governor.
I have always agreed that in theory the way interactive works - by taking over the idle loop - is very attractive. I have never managed to tweak it so it would behave decently in real life. Smartass is a complete rewrite of the code plus more. I think its a success. Performance is on par with the "old" minmax and I think smartass is a bit more responsive. Battery life is hard to quantify precisely but it does spend much more time at the lower frequencies.
Smartass will also cap the max frequency when sleeping to 245Mhz (or if your min frequency is higher than 245 - why?! - it will cap it to your min frequency). Lets take for example the 998/245 kernel, it will sleep at 245. No need for sleep profiles any more!

Source:xda