Newsletters are used to keep an active presence with your website visitors. A newsletter will remind a subscriber that you, your website, and your service continue to exist.
Perhaps you've heard of the old adage made new again: "Out of site, out of mind." There are many factors which bring a visitor to your website. Fundamentally they believe your site has something they want. If you did not have what they wanted then reminding them of your site will serve no purpose. However, if a visitor has joined your newsletter email list chances are good they did find what they were lookign for.
When someone joins a mailing list you can rest assured that either a stranger is pulling a prank and submitted another's email address, or that visitor was moved enough by your site to want to stay in touch. That's a good customer.
A daily newsletter is a bit ambitious. It's a lot of work to prepare new material each day and you may be putting a strain on your subscribers good nature by emailing every day.
A weekly newsletter is very doable as a creative schedule and most people don't seem to mind weekly emails.
A monthly newsletter is the least frequently you should consider as a schedule for newsletters. The only exception to this might be a seasonal site. However, even a seasonal business can benefit from a monthly contact with potential customers.
Keeping in mind that the key purpose of a newsletter is to keep an active presence with your visitors you should make certain that your newsletter reflects your website. A visitor pleased with a website devoted to dogs and dog grooming will not appreciate a newsletter dealing with computers or geneology software.
Every website should have a log-line -- a statement of purpose. This log-line becomes your guiding light and your yard stick with which to measure everything. Before posting a newsletter you read the log-line and ask yourself: "In which way does my newsletter server ...(insert log-line here)?"
If you can't answer this question redo the newsletter until you can.
Outline of a good newsletter:
Any other elements to a newsletter are cream on the peaches. You could include contests, discounts, what have you. These are extra and depend on the season and your marketing plans. Just don't forget the essentials in the mad dash toward a sales hook.