Both are not the same...

Cover letter is use if you want to request a position in a company, make sure you point out your expertise, your objective to apply for the position...

Resume or called "Summary on Yourself Letter" is the summarize list of your working experiences, objective, skills and etc. This is important as your potential employers wil decide whether to hire you or not based on this letter.

for better chance to hire, better send both

You can find to write proper cover letter through internet. There's a lot of ways...its up to you decide.

How about you've already join and working in this company for 3 months now. You have an interview from another company. They want your latest 3 months payslip. Do you have to show them your current payslip, or your previous payslip? In the resume you don't mention your currrent work because its just 3 month only. How do you clarify on this issue?