capital gains tax

I was talking with my realtor this evening. She told me that there was a cap. gains tax if i sold a property too soon. I thought that you only paid that if you made some extremely large amount of money on the sale of a house. Does my realtor have a point? Should there be something in the contract about this? What types of things should there be in the contract to prevent things like this from happening? Thanx, kmb4701 :)