Sharpness and blur can be added to your pictures globally, but should you wish to add these affects to just a small part of an image you would normally have to create a selection, feather the edge of that selection and work within it.
An alternative is to select the sharpen or Blur tool as shown below.
Like the paint tools you can choose a brush size of your choice. In Photoshop 6 these options will appear at the top of screen when the tool is selected and will be displayed as shown below.
Photoshop version 5 users will need to double click the icon in the toolbar for the option palette to appear.
Generally the pressure setting you should use for these tools is likely to be low. Start around the setting of ten, but be prepared to reduce that if the effect is too great. Remember that a subtle approach is generally the one that will work best.
The smudge tool is set up in the same way and best used with a low pressure setting and will allow you to soften the edges of objects within your picture space. The example below shows the effect of this tool.