There are two kinds: one that is invalid due to difference of opinion in it while being executed by the Imam for example as in the case of marrying without the consent of the guardian but with the approval of the Islamic judge or the ruler.
The second type is void and did not take place in the first place as in the case of a marriage without any witnesses.
The majority of scholars say that they are the same.
Others say that it is different as the first one has to have a divorce to get out of it while the other marriage didn’t happen in the first place.