お世話になっております

If you're in Japan, this sentence will probably rank top 5 frequent sentences. The first sentence from the email from someone you never met, they will open with:

お世話になっております。

What exactly have I done to you that you have to say:

お世話になっております。

？？？？？？？？

世話

If you look up 世話, it has 6 meanings:

• （１）（人や生き物に対して）気を配って面倒をみること。手数をかけて援助すること。
  Care for someone's trouble.
• （２）中に入って取り持つこと。仲介。斡旋。
  Mediator.
• （３）手数がかかって苦労すること
  Spent time doing something for someone.
• （４）日常的なこと。卑近なこと。
  Daily, common things.
• （５）世間でよく口にする話や言葉。世間の言いぐさ。下世話。
  Words popularly speak in the word.
• （６）世俗で使う言葉。俗語。話し言葉。
  Gossip.

The（２）,（４）,（５）, and（６）are already not in used. Nowadays it usually means （１） and （３）.

Examples

When you meet someone at work, お世話になっています。

Thanks for taking care of me.

Altough you haven't done anything for them. They said this in a way to hope in the future you would do something good for them.

When you call a business, お世話になっております。

Thanks for calling. Thanks for (taking the time) calling.

When you finished a job, you say お世話になりました to everyone:

Thanks for everything (you have done for me).

When mom telling you that your aunt tried to lump you with someone:

A: おばさんだって、あなたのことをご心配して。。。

B: 余計なお世話です

Not your business.

子供の世話で忙しい
taking care of children.

世話をかけて申し訳ない
Sorry for causing you troubles.