When it comes to the top smartphones in the United States (U.S.) throughout the last year, Apple Inc.'s iPhone brand proved No. 1.

According to the NPD Group's Mobile Phone Track, which provides advisory and information services worldwide, Apple's iPhone maintained its title as the top smartphone brand in the U.S. for a consecutive year.

Although The NPD Group did not specify specific iPhone models, such as if the iPhone 5s outsold the iPhone 5c, iPhone 5, or iPhone 4s, the Apple smartphone easily commanded the competition with a 19 percentage-point lead against the second place rival. Apple accounted for 45 percent of the U.S. smartphone brands market share for 2013. In comparison to 2012, Apple garnered 44 percent of the market share and a 20 percent lead from its second-place rival.

The NPD Group acknowledged one-third of iPhone buyers were considered "more affluent consumers." For low-income consumers, reportedly earning less than $30,000, iPhone sales grew by 64 percentage points. While iPhone sales grew among the low-income buyers, the sales rate by affluent consumers fell from 44 percent in 2011 to 33 percent in 2013.

Samsung smartphones sales were enough to finish at second place for another year. Although specific Samsung smartphones were not disclosed, the Galaxy S4 company represented 26 percent of the U.S. smartphone brands market share. For Samsung, the aforementioned market share percentage is an increase of 2 percent from 2012's 24 percent.

Samsung's smartphone sales were largely as a result of the under $30,000 consumer, which represented 35 percent of the company's sales. The "more affluent" consumers apparently didn't buy into Samsung as much compared to Apple. The affluent consumers, noted to earn more than $100,000, accounted for 18 percent of Samsung sales during 2013.

"With the fastest growing segments of the industry in the lowest income demographics, both Apple and Samsung face challenges in 2014," said NPD's Vice President of Industry Analysis Stephen Baker. "For Samsung this demographic is likely to be the most competitive segment of the market in 2014 and they have a very high dependency on sales here. Apple has the opposite problem of gaining share in the fast growing entry-level market while still maintaining its position as the dominant supplier to affluent consumers."

LG, HTC, and Motorola rounded the top five-smartphone brands below Samsung with 8 percent, 6 percent, and 4 percent, respectively.

The NPD Group stated 2013 was a year of "smartphone market stability" in the U.S.

"Overall industry growth was similar to that of 2012, and while the major hardware brands saw their shares increase marginally, the space between Apple and Samsung and the rest of the industry expanded once again," added Baker.

In general, the U.S. consumer smartphone market grew by 21 percentage points in 2013 as more than 121 million smartphone units were sold. The sales growth during the last year was attributed to the surge in prepaid device, which increased by 68 percent and represented 29 percent of the total smartphone market. Postpaid smartphone sales dipped to 71 percent of the overall market share following a growth of less than 10 percentage points.

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