It's already been covered but I consider an alliance as being an ALLY. Another gang you can count on to come to your aid in time of need in vice versa.
In addition, there is no need to list allies on your gang's description page. IMO it's ridiculous. There's no reason to broadcast who you are or not in an alliance with. The aggressor will find out who the aggressee is in an alliance with if it gets to that point.
Personally I think it's quite humerous all of the gangs that have 30 gangs listed as their alliance partners. The only thing more funny than this is when they list...
alliances:
x gang
y gang
z gang
and so and so in xxx gang(insert person's name who is in a gang but that gang is not on the list) lol
True members of alliances know who is in their alliance and therefore it does not need to be written in your description.