Css – Specific media queries for target mobile (first), tablet & desktop

IMO these are the best breakpoints:

@media (min-width:320px)  { /* smartphones, portrait iPhone, portrait 480x320 phones (Android) */ }
@media (min-width:480px)  { /* smartphones, Android phones, landscape iPhone */ }
@media (min-width:600px)  { /* portrait tablets, portrait iPad, e-readers (Nook/Kindle), landscape 800x480 phones (Android) */ }
@media (min-width:801px)  { /* tablet, landscape iPad, lo-res laptops ands desktops */ }
@media (min-width:1025px) { /* big landscape tablets, laptops, and desktops */ }
@media (min-width:1281px) { /* hi-res laptops and desktops */ }

Edit: Refined to work better with 960 grids:

@media (min-width:320px)  { /* smartphones, iPhone, portrait 480x320 phones */ }
@media (min-width:481px)  { /* portrait e-readers (Nook/Kindle), smaller tablets @ 600 or @ 640 wide. */ }
@media (min-width:641px)  { /* portrait tablets, portrait iPad, landscape e-readers, landscape 800x480 or 854x480 phones */ }
@media (min-width:961px)  { /* tablet, landscape iPad, lo-res laptops ands desktops */ }
@media (min-width:1025px) { /* big landscape tablets, laptops, and desktops */ }
@media (min-width:1281px) { /* hi-res laptops and desktops */ }

In practice, many designers convert pixels to ems, largely because ems afford better zooming. At standard zoom 1em === 16px, multiply pixels by 1em/16px to get ems. For example, 320px === 20em.

In response to the comment, min-width is standard in "mobile-first" design, wherein you start by designing for your smallest screens, and then add ever-increasing media queries, working your way onto larger and larger screens.

Regardless of whether you prefer min-, max-, or combinations thereof, be cognizant of the order of your rules, keeping in mind that if multiple rules match the same element, the later rules will override the earlier rules.

There are a lot of medias out there, and if you want to select only by its properties, use the all keyword:

@media all and (max-width:480px)
{
    /* Styles */
}

Edit:

Combine rules with or:

@media all and (prop1:val1), all and (prop2:val2)
{
    /* Styles */
}

Combine rules with and:

@media all and (prop1:val1) and (prop2:val2)
{
    /* Styles */
}