Sometime a picture suffers from a blur created by the camera shaking when taking a photograph. The lines are, therefore, not quite sharp. To minimise blurs, open picture in Photoshop. In Images, go to Filter and then to Sharpen. You can use the options of Unsharp Mask and Smart Sharpen to take care of the blur. If you want to sharpen some parts of the picture and not the whole, then go to Layers (F7), Duplicate Layers (Ctrl+j), then sharpen the upper layer as much you want. You can then use the erase tool (press E ) to rub away the portions that you do not want to sharpen.
On the contrary, if you want to blur out certain portions of your picture, you can follow the same procedure as for sharpening parts of the image. Instead of the Sharpen option, just use Blur.
An easy way to use blur or sharpen is to go to the Blur tool (waterdrop-like icon) on the left menu. Clicking on its give you the options for blur, sharpen and smudge. Choose the area on the picture you want to work on and use the tool.