They are spelling variations of the same word.
The old spelling rules (before the 90s) specified that the verb to be should be conjugated like this:
sînt (I am / they are), sînteți ( you are - plural), sîntem (we are).
Sometime in the 90s the Romanian Language Academy (I'm not sure of it's official name), changed the rules (it's a more complicated topic), and among the new rules, the verb "to be" became "sunt" (I am/they are).
The funny thing is that everyone still pronounces this word as ”sînt” (except news anchors, but they don't have a choice)
There are still a lot of people who refuse to adhere to the new spelling. So you'll still occasionally see both forms.
On the good side, if someone sends you an email and they write "sînt" you'll know for sure they graduated highschool before the 90's :)