Allyoop is correct. Channel stitching is rows of stitching parallel to each other, often done on a LA with a locking mechanism engaged on the machine rails (called channel locks) so there is no wavering on the stitch lines.
SITD is quilting in the "ditch" formed by seams. It is not bad for your quilt to SITD. However, if seams are pressed open and the quilter stitches exactly in the ditch (as opposed to just a hair over) they are not catching any fabric on the top and only quilting over the thread used to do the piecing. The risk is that in quilting the LA needle could hit one of your seam threads dead on and break it, theoretically compromising the integrity of the piecing. However, most LA quilters will realize you have pressed your seams open and will either advise against ditching or ditch a hair over.
The difference between the two are also cost. SITD is very expensive because it is so labor intensive. Channel stitching is not. But your quilt top needs to be nice and square for channel stitching to look good. An all over pantograph, also known as an end to end (E2E) is also very economical option for quilting and if your fabrics are busy it is usually a very good choice and done with a thick polyester shows off the design very nicely.