Momentum is a construct of physics defined as the product of mass and velocity. Although it is just a construct, it is very useful as it has some very interesting properties, like being conserved in most interactions. In 4 dimension spacetime employed by relativity, it is also shown to belong with energy and other basic constructs that are conserved in interactions. Mathematically, it is represented by p.
Impulse is simply the change in momentum, and is represented by I when used in mathematics. It is hardly ever used, and mostly only to demonstrate the concept. We will deal with it with forces in the next page.