The Cortex A7 is a new chip that is inexpensive to build, thus make for budget smart phones (less than $100) with performance probably equal to a iPhone 3G or 4.
The Cortex A15 is suppose to come out sometime this year as a dual-core, followed by a quad-core, probably in 2013. The A15 is suppose to be a 5-fold improvement over the A-9.perhaps the most important feature of the A7 is that it can be combined with much higher-power cores like the Cortex-A15 side-by-side on the same chip. This allows a super-phone or tablet to switch between two totally different processing units depending on how much power is needed at the time. ARM calls this "Big.LITTLE" computing," and a similar concept is already in use on NVIDIA's Tegra 3 (aka Kal-El) SoC, which we'll see imminently in the next Asus Transformer. However, the Tegra 3 uses five identical Cortex-A9 cores, whereas a device that mix-and-matches the A15 and A7 could potentially deliver higher highs and lower lows, giving you speed when you need it and amazing battery life when you don't