Check out example codes for "media queries". It will help you in understanding the concepts better.

Code Example 1

@media only screen and (max-width: 600px) {
  body {
	background-color: lightblue;
  }
}

Code Example 2

span { background-image: url(particular_ad.png); }

@media (max-width: 300px) {
    span { background-image: url(particular_ad_small.png); }
}

Code Example 3

/* 
  Extra small devices (phones, 600px and down) */
@media only screen and (max-width: 600px) 
  {...} 

/* Small devices (portrait tablets and large phones, 600px and up) 
  */
@media only screen and (min-width: 600px) {...} 

/* Medium devices (landscape tablets, 768px and up) */

  @media only screen and (min-width: 768px) {...} 

/* Large devices (laptops/desktops, 992px and up) 
  */

  @media only screen and (min-width: 992px) {...} 

/* Extra large devices (large 
  laptops and desktops, 
  1200px and up) */
@media only screen and (min-width: 1200px) {...}

Code Example 4

@media only screen and (max-width: 600px) {
  body {

   
background-color: lightblue;
  }
}

Code Example 5

@media all and (max-width: 699px) and (min-width: 520px) {
   ul li a {
    padding-left: 21px;
    background: url(../images/email.png) left center no-repeat;
  }
}

Code Example 6

/*Extra small devices (phones, 600px and down) */
@media only screen and (max-width: 600px) {...} 

/* Small devices (portrait tablets and large phones, 600px and up) */
@media only screen and (min-width: 600px) {...} 

/* Medium devices (landscape tablets, 768px and up) */

@media only screen and (min-width: 768px) {...} 

/* Large devices (laptops/desktops, 992px and up) */

@media only screen and (min-width: 992px) {...} 

/* Extra large devices (large laptops and desktops, 1200px and up) */
@media only screen and (min-width: 1200px) {...}

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