Easy Answer: The numbers on the bottom of plastic bottles let you know the type of plastic they're made of. The easiest to recycle are numbers 1 and 2.
Not all plastics are created equal or out of the same stuff, so the Society of the Plastics Industry came up with a numbering system to help recyclers figure out what bottles and containers are made of. You've seen these numbers before -- they're often on the bottom of containers and have three chasing arrows around them. (Sure does look a lot like the recycling logo, doesn't it?) But, just because plastic has a number DOESN'T mean it can be tossed into your blue bin.
Here's a little cheat sheet to help you determine what's recyclable and what's not.
1 Polyethylene Terephthalate (PET, PETE)
Common uses: Drink and water bottles, food jars
2 High Density Polyethylene (HDPE)
Common uses: Milk jugs, shampoo bottles, grocery bags.
3 Polyvinyl Chloride (PVC, Vinyl)
Common uses: pipe, food containers and shrink wrap
Health concerns: According to the NRDC: "The production and incineration of PVC releases carcinogenic dioxins into the atmosphere. Furthermore, some products, such as certain food wraps, can leach hormone-disrupting phthalates into foods."
4 Low Density Polyethylene (LDPE)
Common uses: Dry cleaning bags, shopping bags, garbage bags, paper milk container coating
Sometimes recyclable. You'll need to check with your community. Some cities require grocers to collect and recycle plastic shopping bags.
5 Polypropylene (PP)
Common uses: Yogurt containers, medicine bottles
Sometimes recyclable. You'll need to check with your community.
6 Polystyrene (PS)
Common uses: Cups, plates, takeout containers
Health concerns: According to the NRDC: "Can leach carcinogenic styrene into liquids."
Common uses: Reusable water jugs, oven-baking bags