Does an invalid marriage for Catholics need an annulment for convalidation or talking with a priest to convalidate?
So, for example, two Catholics who were married by an Elvis impersonator in Las Vegas would be in an invalid marriage due to a lack of canonical form. If later on in life they came to their senses and wanted to convalidate their marriage in the Church they can do so without obtaining an official decree of nullity (i.e. annulment) for their invalid marriage. This is due to the fact that for Catholics a mere civil marriage is, essentially, prima facie considered invalid.
If, however, the couple divorced and one of them wanted to marry another person in the Church then an official decree of nullity would be sought to simply be sure the marriage had never been validated by the Church at a later time.