Adding an Expires header to your website with a date in the future makes the files cacheable, reducing the load time of your pages. This should be done with images, but that's fairly typical. Go a step further and add it to scripts and stylesheets, too. This won't affect performance the first time users hit your page, but on subsequent page views it could reduce response times by 50% or more.
Here's an example configuration for your .htaccess file enabling Expires headers on common file types.
# Set long expire headers for better browser caching
<IfModule mod_expires.c>
ExpiresActive On
ExpiresByType text/css "access plus 30 days"
ExpiresByType text/javascript "access plus 7 days"
ExpiresByType application/x-javascript "access plus 7 days"
ExpiresByType application/javascript "access plus 7 days"
ExpiresByType image/x-icon "access plus 7 days"
ExpiresByType image/vnd.microsoft.icon "access plus 7 days"
ExpiresByType image/png "access plus 30 days"
ExpiresByType image/gif "access plus 30 days"
ExpiresByType image/jpeg "access plus 30 days"
ExpiresByType image/jpg "access plus 30 days"
ExpiresByType application/x-shockwave-flash "access plus 30 days"
</IfModule>
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