Solid State Disk (SSD) is in a non-mechanical disk with no moving parts. Information is stored in microchips whereas a hard disk drive uses a mechanical arm with a read/write head that moves around a spinning metal platter with a magnetic coating to store and read the data, comparable to vinyl record players. In general, but specifically in laptops, SSDs should outlast any hard drive they replace as there are no more mechanical spinning disks with an SSD, it is shock resistant up to 1500g so less susceptible to damage and data loss due to bumping or moving it too quickly. 

The transistor basically ushered in the solid state era, not just for CPUs (processing) but also storage (RAM, hard-disks, tapes, DVDROM, etc). Your computer's RAM is made of volatile memory, meaning it cannot 'remember' what was on it once it is un-powered but it is extremely fast. Non Volatile Memory (NVM) retains the data stored when un-powered, like  digital camera SD cards. SSDs and NVMe’s are the latest incarnation which are much faster than HardDiskDrives (HDD)

SSDs plugs into a normal hard-drive SATA or SAS port with the advantage that they are not only much faster i.t.o. throughput, but much quicker in response times. NVMe’s plugs into a server’s PCI-E port and is even faster than SSDs because it does not have to go through the slower SATA port.

Performance is measured by Input/output operations per second (IOPS, pronounced eye-ops) is basically how many IO requests a device can process per second. Latency describes how long it takes for an IO request to complete, and Throughput is the actual speed of the data transfer and most often measured in MB/s.

Throughput: HDD 150MBps  SSD:500MBps NVMe 2000MBps

Latency: HDD=10ms  SSD<1ms  NVMe=nanoseconds

IOPS: HDD=100  SSD=40 000 NVMe=450 000

It is unfortunately not a one-size fits all with SSDs or NVMe. Manufacturers will give performance specs and lifetime estimates but care should be taken to select the right SSD for the job. Some SSDs are optimized for reading, others for writing and in a server scenario both reading and writing are usually equally important. More expensive SSDs will last much longer and give much more stable and predictable performance under many different types of loads, so is also worth the investment for a server (not so much for a notebook). SSDs are more expensive than HDD per GB of space, but have many benefits that is definitely worth it (unless you need to store Terra-bytes of data.)

Hybrid HDD and SSD storage solutions like ZFS or CEPH have emerged and can combine the best features of both HDDs and SSDs. True Technologies specialises in these solutions.