The auxiliary verb construction used to is used to express an action that took place in the past, perhaps customarily, but now that action no longer customarily takes place:
We used to take long vacation trips with the whole family.
The spelling of this verb is a problem for some people because the "-ed" ending quite naturally disappears in speaking: "We yoostoo take long trips." But it ought not to disappear in writing. There are exceptions, though. When the auxiliary is combined with another auxiliary, did, the past tense is carried by the new auxiliary and the "-ed" ending is dropped. This will often happen in the interrogative:
Didn't you use to go jogging every morning before breakfast?
It didn't use to be that way.
Used to can also be used to convey the sense of being accustomed to or familiar with something:
The tire factory down the road really stinks, but we're used to it by now.
I like these old sneakers; I'm used to them.
Used to is best reserved for colloquial usage; it has no place in formal or academic text.