SSD or Solid State Drive is the alternative for the more traditional hard drives found on laptops and pc's, and there are a number of differences between the two storage devices.
The traditional hard drives found on pc's and laptops have some moving parts which include the spinning disc (where the magnetic data is stored) and an arm that moves around inside (which actually reads the data from the disc)
A SSD has no moving parts though, but it does have a limited amount of times that data can be writen to the device. Because there are no moving parts, it is also silent and accessing the data is much faster than a conventional hard disc. One of the biggest advantages also, is the fact that a SSD normally uses less energy, and is much more robust, being more resistant to external impact such as bumping or dropping which would normally damage traditional hard drives.
Probably one of the biggest manufacturers of SSD's - SanDisk, released their SanDisk Netbook SDHC Removable Flash Memory Card in June 2009. One drawback for SSD's are that until now, they have been fairly expensive compared to normal hard drives.
SDD's will probably become the norm eventually, replacing the normal hard drives we have today, but for that to happen the technology needs to become cheaper. This will most likely happen over time, as new technology continues to improve on, and even replace more traditional technologies.