The ecash software stores digital money, signed by a bank, on the user's local computer. The user can spend the digital money at any shop accepting ecash, without the trouble of having to open an account there first, or having to transmit credit card numbers. The shop just has to accept the money, and deposit it at the bank. The security is provided by a public-key digital signature.
At the moment, all users and shops must have an account at DigiCash's own bank, the "First Digital Bank" at bank.digicash.com. They can withdraw money from the bank, and convert it to ecash. Shops can be started by any ecash user.