They are called boundaries because this is where one crystal interacts with another. The lattice structure does not continue across the interface without mismatch. While there is some lattice, interaction or sharing it is not complete and there are many defects associated with the boundaries. The degree of mismatch determines if the boundary is a high angle boundary (lots of mismatch) or a low angle boundary (very little mismatch) A tilt boundary is an example of a low angle boundary. This is also one of the reasons that diffusion along grain boundaries is so much higher then through the bulk crystal.