Ghost Mode
A ghost will be compared to your gameplay. Like, if u press a local score OR a global score you can either choose Replay Button or Ghost Mode.In gameplay, another cursor appears in your screen, but the cursor moves according to the replay data, that way you can compare how the cursor moves between your gameplay and the replay data. just to be clear that you're viewing a replay but this time you can see your cursor and play the beatmap at the same time.
Practice Mode
Though ghost and practice modes are particularly the same, ghost mode focuses on the comparison between two gameplays, while in practice mode plays your multiple attempts on a beatmap, both are playing with the replay at the same time.in practice mode, osu will record the data every time you play a song, multiple cursors will show on screen, those cursors will play your previous attempts in that beatmap. Like in ghost mode, you can play with the replay (ironic) but it will be many of them.
I found the idea on this video.
The video was made by NinjaVikingFun
EDIT:
Regarding ghost replays i found a similar topic HERE
My ghost idea might be rejected but i havent found a similar topic about Practice Mode (though it still sounds simillar
)