There are different types of flash memory that are used in SSDs, SLC, MLC, TLC. In layman's terms, you can think of them as densities, and the more densely packed the data is the faster you can access it. The tradeoff is as the density increases, the reliability decreases a bit. Manufacturers will compensate and include some extra space that is used for redundancy allowing the drive to compensate for cells that degrade over time.
TLC is the densest and therefore least reliable, and will degrade a bit faster and have a bit shorter lifetime when written to over and over.
The scratch disk is a disk used as temporary storage, whether it be while some software is open, or for a prescribed period where a large data set needs to be accessed. Some software, like Photoshop allows a user to use a scratch disk. This disk holds data that Photoshop used internally while you work. By using two disks in parallel, the software runs more efficiently because it can read/write data from the internal drive while reading/writing output to the scratch disk at the same time.
The bottom line is that any of the Samsung 840 series drives are very nice, as are the Crucial drives. From Crucial, look for the MX100, M500, and M550. All are good drives. The M550 is the fastest and most expensive of the Crucial lineup, but the 500GB (largest size, IIRC) MX100 is equally as fast. The smaller sizes have write speeds that don't saturate the SATAIII, but are plenty fast for daily use.
In any case, a good SSD will make your current machine feel like a whole new machine.
