Used to

let's have a look to this example:

Susan used to smoke when she was twenty years old.
Susan doesn't smoke anymore. (now)
She said, she used to smoke a lot when she was young.




As we can assume from the example above, Susan used to smoke regularly in the past: was an action that she had been doing for a while in her life.
Something used to happen, it happened regularly in the past, but no longer happens.

examples:
- I used to smoke a packet a day but I stopped two years ago.
- I used to live in London, but I moved back to Italy three years ago.


We also use it for something that was true but no longer is:
- She used to have really long hair but she's had it all cut off. 
- My brother used to go to work by bus, but he bought a car two years ago. 

more examples:
- Did you use to play a lot with your toys when you were a child?
- I didn't use to like her, but after we had holiday together, our feelings got better.

Pay attention:
'Used to do' is different from 'to be used to doing' and 'to get used to doing' :

• to be used to doing
  We use 'to be used to doing' to say that something is normal, not unusual.
  I'm used to living on my own. 
• to get used to doing
  We use 'to get used to doing' to talk about the process of something becoming normal for us.
  I've started working nights and I'm still getting used to sleeping during the day.