Solidarity is a general concept well-defined by
Durkheim. He describes it as coming in two
flavors:
Mechanical and
Organic.
Mechanical solidarity arises between people on account of their similarities. Workers, for example, in the specific Solidarity movement mentioned above, had similar needs and thus banded together to meet them. Mechanical solidarity is thus strengthened by perception of external threat. For lesser examples, sports fans and New Jerseyans also exhibit mechanical solidarity: sports fans perceive their team as threatened by another team; New Jersey is unjustly ridiculed often, and so its inhabitants can form a solidarity in proportion to the ridicule when among others. Sometimes this is Granfallooning, but usually it is not.
Organic solidarity arises between people on account of their differences. Usually this is because they are in some form of symbiosis. The reason to stand by something on which you depend is obvious. However, it can exist without true dependence - enjoying something different is enough. The strength of the solidarity is based on the strength of the perception of dependence.
There are hybrid solidarities, where a group embraces a markedly different group on account of an underlying similarity, but the attitudes are still one of difference.