the different numbers represent what type of material or plastic it is made of. Contrary to what some people think that they represent the number of times they were recycled. They confused the triangle and cycle arrows with being recycled.
1 - polyethylene terephthalate or PET
2 - high density polyethylene or HDPE
3 - polyvinyl chloride or PVC
4 - low density polyethylene or LDPE
5 - polypropylene or PP
6 - polystyrene or PS
7 - other