What makes SSDs so special? (and expensive)

An SSD stands for "Solid State Drive", which means there are no moving parts. This has several benefits:

1. No moving parts means less noise
2. No moving parts reduces mechanical complexity. The simpler something is, the less likely it is to break. SSDs tend to be a LOT sturdier than hard disk drives. You can toss them around and it'll still work fine.

Other than benefits gained from lack of moving parts, an SSD also has some other qualities:

1. Very very fast read speeds compared to harddrives. Random read speeds can be comparable to low end DDR3s. So buying SSDs are almost like buying 64 GB+ of RAM. This means booting your operating system, loading programs, etc all tend to be very fast.
2. Not much difference in sequential read speeds. As compared to random read speeds. Hard disks do a good job at sequential reads, but fails miserably (compared to SSDs) in random reads.
3. Durable but has limited "write cycles". An SSD will fail after writing to it some number of times (a huge number, but still not infinite). In theory a HDD will outlast an SSD because HDDs do not have finite write limit. However in practice, HDDs will almost definitely fail before SSDs do because of environmental factors.