It's storage on a chips instead of on a optical harddisk, overall much faster than conventional harddisks.
So, instead of having mechanical disks rotating inside the enclosure there are memory chips inside.
Conventional disks can be damaged much easier by shocks, HDD are a bit like an old fashioned record player, if you hit it the needle hits the record and you hear a screeching noise, if this happens to a HDD it will be permanently damaged but there are sensors inside to prevent this.
SSD don't have any moving parts in them and are much better.Edit: And more expensive/Less cheap.
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