Before OS X 10.7 Lion, the only way to adjust window size in OS X is at one bottom-right corner. With Lion or later, Apple saw the light and implemented window resizing from any edge and corner.
If you hold down the Option key while resizing, the window will resize from the center. You can use this to keep the window centred at its present location while you adjust its size.
If you hold down the Shift key while resizing, the window will maintain its aspect ratio. This is handy when you’re working on photo and video.
You can combine Option and Shift together while resizing. This will resize the window from the center while maintaining the aspect ratio.