To quote the Boeing Manual:
"The path mode controls descent to fly a vertical path which complies with altitude and speed restrictions in the flight plan. The speed mode controls descent at a fixed speed and complies with altitude and speed restrictions in the flight plan."
Both modes use idle thrust, but path mode will induce a level off if there is an "at" or "at or below" altitude restriction. When it gets to its next idle thrust descent point, off you go. Speed descents make no allowances for this, and will continue down to the lowest restriction altitude but maintain crossing restraints (with no level off.)
Path is normally used, but I prefer level change. Much easier!
As for TOD, it computes back from its end of descent point (usually from a selected approach) to your cruise altitude. DRAG REQUIRED appears when there's a bit more tailwind than it expected and a higher speed is required to maintain the profile.
I hope that helps!