With a wide variety of music available, most tracks will be fine for use in osu!, providing that they are not:
Tracks with explicit lyrics can still be used so long as they do not overtly violate the criteria above.
They will need to be identified by a line in the beatmap's description reflecting the sensitive nature of the content. They must also not have the preview point of the map set to a portion of the song that widely broadcasts said explicit content.
This is a mildly complicated issue and what is acceptable for one person is often not for another.
When considering the appropriateness of a given track, think of the 20% guideline—is more than 20% of your track's lyrical content a direct reference to something explicit, suggestive, or otherwise controversial? If so, it may not be appropriate for osu!.
This guideline is not a hard rule, but more of a way to help inform your choices on what might work and what probably won't.
Many musical styles or genres make use of explicit lyrics for artistic or stylistic ends, and while profanity and explicit content isn't outright forbidden, tracks that are vulgar to the extreme or are deliberately designed to shock or inflame others are not allowed.
In the interests of keeping a game about rhythm strictly about rhythm and in accordance with Community Rule #3, content which is deemed excessively political by the administration is disallowed.
We adhere to no one particular "end" of a political spectrum—osu! is strictly apolitical and we want people to enjoy themselves, not start feuds or quabble over politics.
Any beatmap containing a disallowed track will be removed from the website.
Each time your submission is removed, you will be afforded an explanation by the presiding member of the Nomination Assessment Team or Global Moderation Team responsible for the removal with reasoning justifying the removal.
You may contest this with them via PM if you so wish, or seek the opinions of others to substantiate your claim to the track's suitability. If you still cannot find common ground on this front, your claim may be escalated to a consensus vote among the current NAT members, who will collectively decide if your track is acceptable or not.
Excluding severely inappropriate uploads, all users are given a one submission leeway before penalties are handed out due to the uncertain nature of this ruling. After this leeway has passed, users who continually choose to upload inappropriate tracks will face a 42-hour infringement along with the removal of their submission.