What do numbers located inside a recycling symbol on a product mean?

For plastic containers, the numbers indicate which kind of plastic was used to make the containers.

From: http://www.answerbag.com/q_view/1868294

1: Polyethylene Terephthalate (PET)

Containers that usually fall into this category include water and soft drink bottles, salad dressing containers, peanut butter containers and more. Most recycling programs accept these containers.

2: High Density Polyethylene (HDPE)

The HDPE category includes items like milk jugs, detergent containers, aspirin bottles, toiletry bottles and trash bags. Often the plastic is ecology vocabulary opaque, and it is recyclable in most areas.

3: Vinyl/Polyvinyl Chloride (PVC)

PVC plastics can be found in food packaging, food wrap and cooking oil containers. These containers are not as easy to recycle as PET and HDPE containers.

4: Low-Density Polyethylene (LDPE)

Bread bags, produce bags, squeezable containers like those used for honey, and some grocery bags are low-density polyethylene.

5: Polypropylene (PP)

Items falling into this category include ketchup bottles, straws, some yogurt containers and some medicine bottles. Recycling programs are expanding to include recycling of PP containers, and many programs already accept these.

6: Polystyrene (PS)

Many takeout cartons, disposable cups and plates, egg cartons and meat trays are polystyrene. Some recycling programs accept polystyrene containers.

7: Other plastic not falling into categories 1-6

Water dispenser bottles are a #7 plastic

The classifications are important because different plastics have different structural properties. Similar plastics can be recycled into many more useful products than aggregates of mixed plastics.