When you make a word plural by adding s, don't use an apostrophe.
CORRECT: words, apples, peas, the Joneses, CDs, 1950s.
See? It's wrong to write apple's, pea's, CD's, 1950's or PC's.
That apostrophe is just downright wrong. People rudely call it the greengrocer's apostrophe, because it's often found in the greengrocer's shop. It even looks messy.
Knowing this rule will make life easier, won't it?
Not convinced? Look again at PCs. Big PC in capital letters, and little s in lower case. It's obvious what this means. Nobody would get confused, reading the word PCs.
Just occasionally, it's OK to use an apostrophe with some abbreviations. But this is necessary very rarely.
Examples of when an apostrophe is permitted:
Mind your p's and q's. (ps resembles P.S. in a letter)
Dot the i's and cross the t's. (is would be confusing)