The proposed XL Pipeline will bring oil from Canada's tar sands to refineries in the southern US. From there it will be sold in the US and exported at a profit to the US. Building the pipeline will be a boon to the (unionized) construction industry. Oil pipelines are nothing new or extraordinary. Most of the oil in this country is moved around via pipeline and there is nothing about the XL pipeline to make it riskier than the other pipelines. The pipeline would also ease relations between the US and Canada which have been strained since Obama became president.
There is opposition to the pipeline. Most of it is based on global-warming guru James Hansen who says that if we allow the tar sands to be refined then it will be "game over" for the planet. There are several problems with this statement. One is that it would take hundreds of years to burn all of the Canadian oil. Another is that Canada says that it will allow China to build a pipeline and refine the oil instead.
The Obama administration wants to please both the construction unions and the environmentalists so it put off the decision on the pipeline until after the election. The excuse is that time was needed for an environmental impact study on alternate routes.
Republicans in Congress are in a hurry for the pipeline to be approved so they tried to force the President's hand and demanded a decision. Obama obliged, sort of. He rejected the pipeline because the environmental impact study has not been completed but allowed that the pipeline could be resubmitted and would be judged in due time. This is a double triumph for the Obama administration. They get to tell the environmentalists that they killed the pipeline while hinting in private to the unions that the pipeline will be approved when it is resubmitted.
This is typical of the way the Obama administration has conducted itself since September. Nothing happens unless it helps the Obama reelection campaign. There is no question that the pipeline would create jobs but the Obama administration would rather score political points than help people.