@Christina ***
if you have a single entry tourist visa you'll have to apply for a new tourist visa when you go to a neighboring country. Easier in some than others.
If you are able to get a multi-entry tourist visa before coming to Thailand then you can used it unlimited number of times within 6 months that it's valid. So people can do 60+30, leave and return, 60+(less than) 30, leave and return right before it expires for the final 60+30.
Not every consulate sells multi-entry tourist visas though.