The Act! database folder contains the Act! database file (.ADF), the Act! log file (.ALF) which is the internal transaction log for the Act! data, and the indicator or "pointer" to the database file (.PAD). The .ADF and .ALF must always exist, and must exist in the same folder to comprise the SQL Server database.
Another file type that is associated with the .adf is the remote database file (.RDF). The .RDF is automatically created when a remote database is created for synchronization. The file itself is not a database, but contains the .adf file which is transported to a remote user. Think of the .RDF as a .zip file that must be unpacked and restored to a remote user's computer to use and synchronize Act! data.