The title tag is probably the most important to get right. Each page on your website should have its own title, and should be descriptive of the content in the page. Many websites do not have a title tag, or use the same title on all pages.
Unfortunately, this means that search engines may give the page a lower ranking, browsers won't be able to correctly display the page information in the window header and the site will not conform to standards. A good format for titles is Category - Description, or Brand - Description.
Try to keep the title tag length to only a few words or about 60 characters