In UML, composition is depicted as a filled diamond and a solid line. It always implies a multiplicity of 1 or 0..1, as no more than one object at a time can have lifetime responsibility for another object.
The more general form, aggregation, is depicted as an unfilled diamond and a solid line. The image below shows both composition and aggregation. The C++ code below shows what the source code is likely to look like.
参考：http://en.wikipedia.org/wiki/Object_composition
简单来说：
composition组合，是实心菱形，个体不可以脱离整体存在，生命周期一致
aggregation聚合，是空心菱形，个体生命周期跟整体的可以不一致