TO POP IN(TO) :  Make brief, casual visit, enter quickly
– I just thought I’d pop in and say hello!!                                                                 
– Whenever you are in the neighborhood, you’re welcome to pop in

TO POP OUT :    Go out briefly
– I am just popping out to the shops, I will be back  in 10 minutes                                                                                                                                                  
TO POP ROUND / TO POP BY: to visit someone for a short time – Sarah: Why don’t you pop round for a coffee on your way home from work?
– I haven’t seen my sister in a couple of weeks. I’m going to pop by for an appetizer this weekend and hear the latest gossip.

TO POP UP:  to appear (sometimes suddenly or unexpectedly) – If you click on this button on the website, another window will pop up.
– It’s only November but lots of Christmas advertisements are popping up all over the place