There are sooo many different types of RAM out now. But, what I'll do is give a basic break down of the types that are common for computers today
SDR SDRAM (Single Data Rate Synchronous Dynamic Random Access Memory)
SDR SDRAM isnt used anymore for new computers, instead it was used for slightly older computers (PIIs, PIIIs). The RAM stick itself has 168 pins. Common speeds that are found for SDR SDRAM were PC100 and PC133, meaning that it is matched to a CPU with a 100MHz or with a 133MHz FSB.
This RAM technology is obsolete now, and prices have skyrocketed.
DDR SDRAM (Double Data Rate Synchronous Dynamic RAM)
This is the standard today for new AMD computers. New Intel's have done away and favored for DDR2. DDR uses 184 pins.
The most common speed for DDR is PC3200, with other common speeds being PC2100 and PC2700. Unlike SDR, DDR's PC number corresponds to the bandwidth it provides rather than the actual frequency the chips run at. This means that PC3200 provides bandwidth of 3200 MB/s.
The frequency that the chip runs at is "DDR 400" or 200MHz. Since DDR RAM transfers data twice for every clock cycle (1MHz), it is considered to have a DDR frequency double that of its actual running frequency.
DDR2 SDRAM
DDR2 is currently the standard for new Intel computers. DDR2 is important for Intel because it can supply a lot of bandwidth to their frequency-intensive CPUs.
DDR2 has a lot in common with DDR, but I'll start with the differences. DDR2 now has 240 pins. DDR2 more commonly runs at higher frequencies than DDR does. The higher frequencies are due to an improved electrical interface, on-die termination, prefetch buffers and off chip drivers (thanks wikipedia).
One problem that has come with making DDR2, is latency. Although the speed that the chips can sustain is higher, high latency has hindered its performance a little.
Common speeds of DDR2 are PC2 3200 (DDR2 400), PC2 4200/4300 (DDR2 533), and PC2 5300/5400 (DDR2 667).
Dual Channel
First off, dual channel is not a particular type of RAM. DC is a feature that comes with chipsets (and therefore motherboards). Currently, the more popular sockets support dual channel. AMD's skt939 and AM2 support DC. As well, Intel's skt478 and 775 support DC.
To run RAM in dual channel configuration, you need 2 sticks that are similar or 4 sticks that have similar pairs. When deciding to go with DC, you should get RAM that is identical, as to prevent any problems. However, you can often run non-identical RAM in dual channel. The pair of RAM sticks must be the same size (1GB, 512MB, etc).
Dual channel doesn't give a large performance increase. Overall, it gives about a 1-4% increase in performance.
Other
There are other types of memory available today like RDRAM, but it isnt very common, and pretty expensive.
Hope that helps
If anyone has anything to add, then I'll edit it in. I'm gonna try to get this stickied. I would especially like it if someone wrote up something for latencies
Elbatrop1's Basic Guide to RAM 
Linear Mode
