Three generations of Moto X phones have been released, in 2013, 2014, and 2015. A distinguishing feature is the MotoMaker option, where back and trim colors and materials can be custom-ordered. The 2015 model is called Moto X Pure Edition in North America and Moto X Style in the rest of the world.
The first-generation Moto X was launched August, 2013 running Android 4.2.2 and is available on multiple carriers, including AT&T, Verizon Wireless, Sprint, T-Mobile, and U.S. Cellular in the US. It was subsequently upgraded to Android 4.4 KitKat.
The second-generation Moto X was launched in 2014 running Android 4.4.4. KitKat, with a 5.2 inch AMOLED display, 13MP rear camera (2MP front), and runs a 2.5GHz Qualcomm® Snapdragon 801 with quad-core CPU.
The third-generation Moto X, called the Moto X Pure Edition in North America and Moto X Style in the rest of the world, was launched September 2, 2015, running Android 5.0.1 Lollipop, and was updated to Android 6.0/6.01 Marshmallow in late 2015 to early 2016, and subsequently upgraded to Android 7.0 Nougat in the summer of 2017. It came equipped with a 5.7 inch 2560 x 1440 (520 ppi) QuadHD IPS TFT LCD display with Corning Gorilla Glass 3, 21MP rear camera (5MP front), and runs a 1.8 GHz 64-bit Snapdragon 808 Hexa-core SoC with 3GB of RAM.
In mid-2017, Lenovo's Moto announced they will release a 4th Generation Moto X later in the year named Moto X4.
Note: Unless your question is very specifically about the Moto X hardware, please do not use a device-specific tag.