The way I've always looked at a cover letter is that it forms a partnership with the cv. So the letter is effectively the sales pitch to the employer (why you should hire me), and the cv is the evidence to back it up (I realise a resume shouldn't be thought of the same as a cv, it's more a point list rather than the detail of a cv).
So I have 3 sections as part of my "pitch":
- Intro paragaraph, mentioning the role I'm applying to and a couple of sentences detailing why I'm the ideal choice.
- Second part is a bullet list of relevant points to demonstrate the statement
- Third part another paragraph where I can mention other relevant info, why this is the natural next move for me, and invite the reader to contact me to arrange the next step.
If you have a short resume, you may need to bulk out the evidence in the letter some more (people suggest one page for a resume, my cv usually runs to about 6 pages, even when it's tailored with only relevant projects, but I've been doing this for 20+ years).