Greasers treat their fellow greasers more like family--like brothers--according to Ponyboy's and Johnny's assessments in Susan E. Hinton's novel, The Outsiders. Greasers are poorer and wilder than their enemies, the Socs. Greasers are more like hoods, and they seem proud of this distinction and their proudest possessions--their toughness and long hair. But it is their family-like unity which sets them most apart. Johnny Cade tells Cherry that
"When you're a gang, you stick up for the members. If you don't... make like brothers, it's not a gang anymore. It's a pack..."
The gang to which Pony and Johnny belong do treat each others as brothers, sharing their homes, food and dreams.
There is not really any such thing as a Greaser code of loyalty that is actually specified in the book. It's not like there is a place in the book where it says "The Greaser code of loyalty says..."
However, we can infer some things about it. Clearly, the Greasers are willing to stand up for each other. They come to Pony's aid when he is jumped by the Socs, Dally helps Johnny and Pony get away after they kill Bob, things like that.
I do not know that it really sets them apart. The Socs seem to have a similar code. But it does at least set them apart from people who are not part of gangs -- who don't have a group that will care about them like that.