Relations between Grand Lodges are determined by the concept of Recognition. Each Grand Lodge maintains a list of other Grand Lodges that it recognises. When two Grand Lodges recognise and are in Masonic communication with each other, they are said to be in amity, and the brethren of each may visit each other's Lodges and interact Masonically. When two Grand Lodges are not in amity, inter-visitation is not allowed. There are many reasons why one Grand Lodge will withhold or withdraw recognition from another, but the two most common are Exclusive Jurisdiction and Regularity.
What, besides Recognition, must happen between two Grand Lodges in order for them to be considered in amity? 
Masonic communication