This is the first time since 2011 that Samsung has not been on top of Gartner's worldwide smartphone sales list on a quarterly basis, and it's most likely due to high sales of Apple's iPhone 6 and iPhone 6 Plus. Gartner stated:

"Samsung's performance in the smartphone market deteriorated further in the fourth quarter of 2014, when it lost nearly 10 percentage points in market share," said Anshul Gupta, principal research analyst at Gartner. "Samsung continues to struggle to control its falling smartphone share, which was at its highest in the third quarter of 2013. This downward trend shows that Samsung's share of profitable premium smartphone users has come under significant pressure."

Overall, Samsung still sold more smartphones than Apple for the entire year of 2014, with 307.6 million units worldwide. That number is well ahead of Apple's, which sold 191.43 million smartphones last year.

Source: Gartner