ToC and ICC had 4 difficulty modes.
10 man normal
10 man heroic
25 man normal
25 man heroic
Each was tuned differently and rewarded different ilvl gear (10 man heroic = 25 man normal in terms of ilvl but not actual gear pieces)
In Cata, blizzard made it so that normal mode dropped the same things regardless of player size, same with heroic. This caused a problem as some fights were easier with 10 people, or with 25 people. Basically blizzard removed the player size being part of the difficulty.
So in MoP, blizzard experimented with a flex raid system and it got favourable reviews. So they turned the flex difficulty to normal mode, old normal became heroic (both of which are flex), and old heroic into mythic (20 man by default).
Really, what blizzard did in the WoD pre-patch was basically return to the ToC / ICC style somewhat.
So if you compare them
10 man normal = LFR
25 man normal = normal
10 man heroic = heroic
25 man heroic = mythic