If something used to be done or used to be the case, it was done regularly in the past or was the case in the past.

If something used not to be done or used not to be the case, it was not done in the past or was not the case in the past. The forms did not use to and did not used to are also found, especially in spoken English.

If you are used to something, you are familiar with it because you have done it or experienced it many times before.

If you get used to something or someone, you become familiar with it or get to know them, so that you no longer feel that the thing or person is unusual or surprising.