Charges Attract and Repel
When you rub a glass rod against a balloon, some electrons flow from rod to balloon. The balloon gains some electrons and become negatively charged. The rod loses some electrons and becomes positively charged.
When you hold the rod near to the balloon, the balloon is attracted to the rod. Notice that the rod and balloon have charges of opposite signs - one is positive, the other is negative.
When you rub a glass rod on two balloons, both balloons gain some electrons and become negatively charged. If the two balloons are close together, they repel each other.
Notice in this case that the balloons have charges of the same sign - they are both negative.
We can summarise these observations:
"Like charges repel, unlike charges attract."