背景画像の繰り返し方法を指定します。

repeat, no-repeat, space, round をひとつのみ記述した場合は、横方向、縦方向双方に適用されます。ふたつ記述した場合は、横方向、縦方向の順に適用されます。

background-repeat: no-repeat;         /* 横:no-repeat、縦:no-repeat */
background-repeat: repeat no-repeat;  /* 横:repeat、縦:no-repeat */

.test-repeat {
  background-repeat: repeat;
  background-image: url(./image/back.gif);
  width: 85px;
  height: 85px;
  border: 1px solid gray;
}
.test-repeat-x {
  background-repeat: repeat-x;
  background-image: url(./image/back.gif);
  width: 85px;
  height: 85px;
  border: 1px solid gray;
}
.test-repeat-y {
  background-repeat: repeat-y;
  background-image: url(./image/back.gif);
  width: 85px;
  height: 85px;
  border: 1px solid gray;
}
.test-no-repeat {
  background-repeat: no-repeat;
  background-image: url(./image/back.gif);
  width: 85px;
  height: 85px;
  border: 1px solid gray;
}
.test-space {
  background-repeat: space;
  background-image: url(./image/back.gif);
  width: 85px;
  height: 85px;
  border: 1px solid gray;
}
.test-round {
  background-repeat: round;
  background-image: url(./image/back.gif);
  width: 85px;
  height: 85px;
  border: 1px solid gray;
}

<h5>repeat</h5>
<div class="test-repeat"></div>
<h5>repeat-x</h5>
<div class="test-repeat-x"></div>
<h5>repeat-y</h5>
<div class="test-repeat-y"></div>
<h5>no-repeat</h5>
<div class="test-no-repeat"></div>
<h5>space</h5>
<div class="test-space"></div>
<h5>round</h5>
<div class="test-round"></div>

• http://www.w3.org/TR/css3-background/#the-background-repeat
• https://developer.mozilla.org/ja/docs/Web/CSS/background-repeat
• http://caniuse.com/#search=CSS%202.1%20properties