To me, the covering letter is your 'advert' to entice the employer into reading your application. So mine generally has an introductory paragraph summarising me and my general experience and any key points I want employers to know about me. Then a second para (and third if necessary) referring to specific skills or experience that i have that i know they are looking for, with an example or two demonstrating this.
I do a different letter for each position I apply for.
Keep it short and to the point. Dont waffle, and don't go onto a second page.
Most job applications require a particular form not CV. Look at the person specification and job specification and highlight anything in your experience , skills, training etc that is especially relevant or interesting. Also your motivation for applying - career progression , full time hours, knowledge of company. Keep it succinct and try not repeat things that are self evident in your application.