A library database is an online collection that the library pays for (which means you cannot find it for free on the Internet) and contains a variety of sources that are trustworthy, authoritative, and reliable. For example, it can contain books, peer-reviewed articles, and reports about an issue such as International Security. There are also databases that are comprised of only one type of resource, for example, news or image databases.
It usually has more advanced features than the library catalog that can help you find and organize your materials better. For example, you can browse based on a subject or date, and create automatic emails that let you know when a new article has been published.