This is kidnapping, but with your data. They tell you that you can have it back if you pay, but there is no real guarantee that you will get it back if you pay the ransom. This is a favorite among hackers because it is cheap, effective, and the payout can be huge.
When fishing you are looking to catch any fish, but when spearfishing you are looking to catch one fish in particular. Spear phishing works the same way. Rather than casting a wide net, the phisher tailors their attack to you by using social engineering and any personal information they can find.