A vertex loop is a path of vertices that are connected in sequence by their shared edges. For example, if you select one vertex on a sphere, a loop selection selects all of the vertices along the same line of latitude or longitude on the sphere by selecting each successive vertex that is adjacent to the selected face.
Vertex loop selections let you select several vertices across a polygon mesh without having to select each vertex individually.

There are several options for selecting a vertex loop on a polygonal mesh in Maya. You can select:
To select a vertex loop on a polygonal mesh
over a polygon mesh and select Vertex from
the context sensitive marking menu.
+
double-click a vertex connected to an adjacent edge in the direction
you want the vertex loop.
To select a partial vertex loop on a polygonal mesh
over a polygon mesh and select Vertex from
the context sensitive marking menu.
+
double-click a non-adjacent vertex on the same line of longitude
or latitude.