Y-seams and Inset seams are the same thing.
they're used when two pieces don't fit together along one straight line. quilts made with hexagons, for example, are done mostly using Y-seams.
you can try to sew a continuous Y-seam by stitching from an outer edge into the point and then fiddling with and pivoting the fabrics so you can sew back out to the opposite edge. that's fairly easy to do if you're piecing by hand. it takes practice if you want to do it by machine.
another way to deal with Y-seams is to sew from one edge into the point and finish with a backstitch. then sew from the other edge into the point using a separate seam - also ended with a backstitch.
it will look the same no matter which method you use.
here are two tutorials that use the two-seam method. the first demonstrates assembling the "flowers" in a Grandmother's Garden quilt from hexagons. the second shows how to attach a triangular piece that "tucks into" the adjoining piece.
http://www.quiltingboard.com/t-3419-1.htm
http://www.quiltingboard.com/t-34097-1.htm