These are the most basic systems which rely on outside sounders scaring off an intruder or warning neighbours that there is a problem. This is by far the most common and cost effective type of system.
Basically, this is a bells only system with a piece of equipment called a speech dialler fitted to it. The speech dialler is connected to your telephone line and contacts up to 4 different people should your alarm activate. When it calls you can activate a microphone within your premises to check if the activation is genuine. If it is you can then contact the police yourself. The beauty of this system is that you can fit it to any alarm, no matter how old and not have the burden of expensive monitoring contracts or updating your equipment. This is a piece of equipment that most alarm companies will not tell you about, as they would much rather sell you a new system and a high annual contract. If your insurance will accept this, it is superb and highly recommended.
This is basically the same as a dial up connection on your computer. When the alarm activates it connects via your phone to a monitoring station who pass the call to the police or keyholder depending on the situatuion. This type of monitoring can be interfered with if the line is blocked or cut.
This is the broadband version of alarm monitoring. Your alarm is permanently connected to the monitoring station who are notified instantly should there be a problem, including your lines being cut.
Both work on a principle of having a back up to a basic redcare connection, either via radio link or mobile connection.