I was on chat in Stack Overflow and noticed that some of the message had grey stars with a black border while others were just black stars. What is the difference between the 2 star types?
The filled in stars are regular chat stars. The unfilled stars are pinned chats. Stars hang out for a while and decay, the pins will hang out at the top of the stars list, but will also decay after a while.
Only room owners and chat moderators can pin things.
(quote from comments)
Room owners can also pin messages, which permanently affixes them to the top of the room sidebar for up to 14 days.
The hollow stars are for messages pinned by moderators and room owners. They stick to the top of the side bar so everyone can see them.
The solid stars are for messages starred in the regular manner. These gradually sink as other messages are starred.
If the star is yellow - you starred it.