A database (also called the song library) is a collection of songs. Specifically, it is the contents of one or more folders of song files. A listing of the songs is stored in a file on disk, which helps the program to load and run more quickly than if each song was indexed every time.
A single database can contain more than one folder of songs. You can sort your songs by type if you wish, into different folders. Since you can sort the songs by folder (add the Folder as a column in the library and click on that column), this may be useful in organizing and selecting songs for a worship service.
All the songs in a folder are loaded by a database. You cannot only add a portion of the songs in a folder.
Worship LIVE! can use the same song files from the same folders in different databases. This way, you can have different collections of songs for different purposes, without forcing you to duplicate your song files.