Thermoplastics
Not all thermoplastics melt at low temperatures. Some like, polyester, are used as oven bags. These plastics will melt at temperatures between 200 - 300oC. Warnings on the box suggest temperatures below 200oC should be maintained in the oven during cooking. Click to see a 120kb movie. Notice how the oven bag melts with the heat of a burning match.

A thermoplastic has long polymers that are not held together as strongly as they are in thermosetting plastics. As a result of this, when strong heat is applied to the plastic the polymers break their weak links to each other and commence to move past each other. The plastic is said to be melting.

 
Consider the video on the right. It shows a thermoplastic melting. As heat is applied the long polymers gain energy and break the weak forces holding them together and flow past each other, hence becoming liquid.
The monomers of a thermoplastic. The polymer chains are held together by very weak forces. These forces give way when heat is applied and the polymers slide past each other.