by 
The Curious Bag
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The numbers placed on the bottom of plastic bottles are used for recycling purposes and designate the resin type the plastic contains.
History
The numbering system used on bottles began in 1988 by the Society of Plastics Industry (SPI). Both SPI and the Federal Trade Commission regulate the guidelines regarding the numbering code.
Classification
Each number represents the density and amount of resin in the plastic bottle and ranges from No. 1 to 7. Higher-ranging numbers have more polymer density.
Guidelines
Guidelines prohibit the use of the word "recycle"or "recyclable" anywhere near the placed number on the bottle. Additionally, the number must be placed at the bottom of the container.
Examples
Typically bottled water is a No. 1 designation, usually making it good for reuse at least once. Higher density bottles such as Nalgene bottles, designed for repeated use, are classified at No. 6 or 7.
Other Category
Plastic items with the triangle recycle symbol that indicate "other" instead of a number are made of a combination of Nos. 1 through 6.
Source:
PSLC: What Do Those Recycling Numbers Mean?
Plastic Is Forever: What Do Those Numbers on Plastic Mean?
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