Some maps will never be ranked: they're too long, too hard, too strange, or their mapper moved on. Loved gives the ones players love a leaderboard anyway (scores, but no pp).
Sign in to ask for maps and review requests.
Ranked maps go through the Beatmap Nominators and award pp. Loved maps skip that: players ask for them and the Loved team loves them. They get leaderboards and keep everyone's scores, but give no pp.
Graveyard, WIP and pending maps. Ranked, qualified and already-Loved maps can't be, and neither can maps on the blacklist.
Paste the set's link (from here or osu.ppy.sh) above, pick the mode and say why. One request per map and mode: if someone asked for it already, review their request instead. You can ask for up to 5 maps a day.
Open a request and say whether you're for or against it, and why. You can change or remove your review while the request is open. A request's score is its fors minus its againsts (the person who asked counts as a for), and the best-scored requests are listed first.
The Loved team below, each for the modes on their badge. They read the requests, best-scored first, and love the map or reject the request, always with a reason. Loving a map makes its difficulties in that mode Loved straight away.
A rejected map can't be asked for again in that mode (the team can open the request again). A removed request is taken off the list and can be asked for again later. Blacklisted maps can't be asked for at all.
Its mapper can't update it any more. Only an admin can take a map out of Loved.
Tell someone on the Loved team and they'll blacklist it.
Ask one of the admins.
They decide on the requests; their badge shows the modes they decide for.