Passwords and access lists will provide limited protection but all of your information is still sent "in the clear". This means anyone on a network through which your traffic passes can still read the contents.
The only way to keep your information private is by using encryption. Encryption is a means of converting your data to code. For example a simple way to hide your data would be to UUEncode it. UUEncode is a program that convert files from 8 bit ( binary ) data to 7 bit data. This makes it unreadable unless someone has the UUDecode program.
UUEncoding a file is not really a secure means of encryption, since all that is needed is the UUDecode program. This program is freely available from most FTP sites and is also distributed with all UNIX machines.
Using UUEncode will keep casual snoopers from seeing your information, but a dedicated cracker can save all your information and then run it through UUDecode in order to read it. A better alternative would be to use an encryption product such as PGP (Pretty Good Privacy) or RSA.
These packages require you to encrypt the data separately and then send it. It is not transparent, so if transparency is important, hardware encryption will be required.
Hardware encryption requires a separate device to be attached, either between you and your network, or between the two networks. An example would be an encrypting router, which will automatically scramble your network data so it will be unreadable. By setting up two encrypting devices at opposite sides of the network, you can have transparent encryption.