Eigen defines several typedef shortcuts for most common matrix and vector types.
The general patterns are the following:
MatrixSizeType where Size can be 2,3,4 for fixed size square matrices or X for dynamic size, and where Type can be i for integer, f for float, d for double, cf for complex float, cd for complex double.
For example, Matrix3d is a fixed-size 3x3 matrix type of doubles, and MatrixXf is a dynamic-size matrix of floats.
There are also VectorSizeType and RowVectorSizeType which are self-explanatory. For example, Vector4cf is a fixed-size vector of 4 complex floats.