The Replicators were created at an unspecified time in the past on a Human world where an android called Reese was created. This machine had a child-like personality and was feared by the natives of the planet. In order to keep herself amused, she created the Replicators which were to be her toys. As the population of the world grew more fearful of Reese and the replicators, she began to teach the machines to protect her and themselves. In order to accomplish this, they began the process of self replication. The Replicators grew too large in number, and Reese herself began to lose the ability to control them. After Reese lost control of the Replicators, they destroyed her world and everyone living on it. The Replicators abandoned Reese in pursuit of raw materials, and Reese, feeling guilty and abandoned, went to sleep. She was later discovered by SG-1 and brought to Earth where she was shot while trying to leave Earth with her newly created Replicators. (SG1: 'Menace')
A Replicator being assembled.
'Each individual building block is capable of exerting a reactive modulating monopolar energy field on other blocks allowing the replicators to assemble themselves into many forms. To our knowledge, the interior of each block contains the following; 2 million isolated keron pathways…'
Each replicator is composed of a series of Replicator blocks. Each block is made up of many Keron pathways. It takes several blocks working together to make new blocks. Replicators are capable of forming many patterns determined by what they need to accomplish. Although Replicators usually take the form of 'bugs,' they can take other forms. The blocks are capable of absorbing large amounts of energy, which can empower them. The Asgard's dependence on energy weapons made it difficult for them to develop a weapon which could effectively counter the Replicator blocks. Tau'ri guns were capable of permanently damaging Replicator blocks (at most it would take two or three shots) but Human form Replicators are impervious to them. (SG1: 'Nemesis', 'Small Victories')
The blocks were either capable of being assembled into the different types of Replicators or were formed into other shapes. They were able to create walls of blocks that served as barricades that prevented the enemy from escaping. (SG1: 'Small Victories') Or they were capable of becoming tables and in one case were able to cover an entire planet's surface with blocks which conserved energy. (SG1: 'Unnatural Selection')