Last week I had my hair cut and I also had my car repaired. I don’t know how to cut hair and I don’t know how to repair cars, so both those things were done for me by a trained professional and I paid them for the service.
A few students have asked me since then 'Did you cut your hair?' because in some other languages it doesn’t make a difference who does the cutting. In English I would only say ‘I cut my hair’ or ‘I repaired my car’ if I was doing it myself.
So, just to clarify:
I cut my hair = I cut it myself
I had my hair cut = someone else cut my hair
I repaired my car = I did the repairs myself
I had my car repaired = someone else repaired it for me.
In grammatical terms this is:
Sb + have + object + past participle
'Get' can also be used instead of 'have' to make the expression more informal, as in
'I get my house cleaned once a week by a cleaner'