Definition
Gaussian Radial Basis Function
Similar to the polynomial kernel function, the RBF kernel is a very - if not the most - popular choice to pair with support vector machines. The RBF kernel is much more complex than the polynomial kernel as the feature space is infinite. It is able to combine multiple polynomial kernels of different degrees to project your non-linearly separable data into higher dimensional space. The RBF kernel is usually defined as:
Those kernels, however, were only the more popular ones. You can find kernels for any type of data such as strings or structured data like graphs.