The forty-fourth president of the United States is Barack Obama, he took office in January 2009 and was elected for a second term in 2012.

The forty-fourth president of the United States is Barack Obama, who took office in January 2009 and was elected for a second term in 2012.

The second word group is a complete sentence: It contains an independent clause (The forty-fourth president of the United States is Barack Obama) and a subordinate clause (who took office in January 2009 and was elected for a second term in 2012) joined with a comma. The first word group is a comma splice: It contains two independent clauses joined with only a comma.

For more help, see section G6.