Nokia's low-end Lumia 520/521 series are currently the most popular Windows Phone smartphones on the market. The smartphone maker's introduction of its Lumia 630 and 635 were created with budget smartphone buyers in mind.

Nokia used Microsoft's Build 2014 conference to announce its latest Lumia smartphones that will ship with Microsoft's latest and greatest version of Windows Phone, Windows Phone 8.1. The updated OS brings several welcomed features to Windows Phone 8.1 and puts Microsoft's smartphone OS on a more even playing field with features found in iOS and Android.

For instance, Windows Phone 8.1's centralized notification center makes it easy to see all of your notifications in one spot, just like Android and iOS. Microsoft has also added an action center to this feature that gives users quick access to various settings. Windows Phone 8.1 also introduces Cortana, a virtual assistant that is a cross between Siri and Google Now.

The Nokia Lumia 635 boasts a 4.5-inch FWVGA ClearBlack display with a resolution of 854 x 480, a pixel density of 221 pixels per inch (ppi) and will be the first device to ship with Windows Phone 8.1. The smartphone is powered by a 1.2GHz quad-core Qualcomm Snapdragon 400 processor and a 512MB of RAM. It will ship with 8GB of internal storage and a microSD expansion slot that will support up to 128GB microSD cards.

Its camera capabilities include a 5-megapixel rear camera with Auto focus and 4 x digital zoom. Its wireless connectivity options consist of 4G LTE radio, Wi-Fi 802.11 b/g/n, Bluetooth 4.0, and GPS. The Lumia 635 will ship with a 1,830 mAh removable battery and will be offered by T-MobilE, at&t, AND MetroPCS in the U.S. sometime this summer and is expected to retail for $189.

The Nokia Lumia 630 has virtually the same specifications as the Lumia 635, but will be offered without a 4G LTE radio and instead use a 3G radio to keep costs down and will also include support for dual-SIM cards. The Nokia Lumia 630 is expected to begin rolling out in May internationally and will be priced around $159 for the single SIM model and $169 for the dual-SIM variant.

We'll keep you updated on exact release date and pricing when it becomes available.