The Siamese Cube is a construction where two 3x3x3 cubes are joined together (typically, but it may be any size of cubes, or even other types of puzzles). There are a couple of ways to do this, but what is thought of as the real thing is the Siamese 3, where the number tells how many pieces of the two puzzles that are shared between them. Another common type is the Siamese 6, but that is easier to solve than the Siamese 3.
One of the most effective ways to solve the siamese is to use the Roux Method but also the Fridrich Method works if you add a few more algorithms to the original system. The Petrus Method is often easier because the EO removes the problems that occur when you use Fridrich (the EO has to be done a bit differently, though). In Petrus you can use 2GLL for the last step, or rather, you can use 2-gen for all that comes after 2x2x3 and EO.