A provider is a company that provides the library with access to a product. The product might be a database, index, e-book, or some other online resource. The library purchases the legal right to access the information from the provider's website.
A provider is also sometimes called a vendor and often it is a publishing company (publisher).
The provider determines how the information provided is displayed and search, through the interface it uses.
Sometimes a separate company (eg. EBSCO) buys the rights to various related products from several publishers and sells them as a package.