Difference between short tail keywords and long tail keywords

Hello
What is the difference between short tail keywords and long tail keywords?

Short-tail keywords usually have more searches in search engines, but they are harder to rank since many other people try to rank for them. The example is “Headache.”

Long-tail keywords are easier to rank, but have fewer searches. The example is “Causes of headache in pregnancy.” On the other hand, long-tail keyword tend to have higher click-through rate (CTR) — readers tend to click on ads more when reading more specific (long-tail) than general (short-tail) articles.

Sometimes it is good to start to optimize your site or article for the long-term keywords and bring them to the top of Google, and this way present the site to at least few searchers; better than trying only with long-tail keywords and rank low where rarely one will click.