So you wanna know what this button does before you click on it? Okay! (I tried using pseudo elements for the little nub arrow thingy, but they didn't seem to play well with transitions. Anyone wanna have a crack at it?)

index.html

<div class="wrap">
  <button class="tip">
    What's this do?
    <!-- change the class on the span below between 'top', 'right', 'bottom', 'left' -->
    <span class="left">It doesn't do anything, don't bother clicking on it.<span class="nub"></span>
    </span>
   
  </button>
</div>

style.less

@import url(http://fonts.googleapis.com/css?family=Archivo+Narrow:400);
@import url(http://fonts.googleapis.com/css?family=Archivo+Narrow:700);
@tipcolor: #0099ff;
.default(){
  text-shadow: 1px 1px 0px #fff;
  font-weight: 400; 
  color: #333;
  border: 1px solid #aaa;
  cursor: pointer;
  .rounded(4px);
  background: #feffe8; /* Old browsers */
  background: -moz-linear-gradient(top,  #feffe8 0%, #d6dbbf 100%); /* FF3.6+ */
  background: -webkit-linear-gradient(top,  #feffe8 0%,#d6dbbf 100%); /* Chrome10+,Safari5.1+ */
  background: linear-gradient(to bottom,  #feffe8 0%,#d6dbbf 100%); /* W3C */
  .transition;
  &:hover {
    background: -moz-linear-gradient(top,  #fffce7 0%, #fdfd9f 100%); /* FF3.6+ */
    background: -webkit-linear-gradient(top,  #fffce7 0%,#fdfd9f 100%); /* Chrome10+,Safari5.1+ */
    background: linear-gradient(to bottom,  #fffce7 0%,#fdfd9f 100%); /* W3C */

  }
  &:active {
    background: -moz-linear-gradient(top,  #f8c67a 0%, #e28904 49%, #f9ac39 50%, #f9d4a1 100%); /* FF3.6+ */
    background: -webkit-linear-gradient(top,  #f8c67a 0%,#e28904 49%,#f9ac39 50%,#f9d4a1 100%); /* Chrome10+,Safari5.1+ */
    background: linear-gradient(to bottom,  #f8c67a 0%,#e28904 49%,#f9ac39 50%,#f9d4a1 100%); /* W3C */
  }
}

.textinset(@horizontal: 1px, @vertical: 1px, @blur: 0px, @color: #fff) {
  text-shadow: @horizontal @vertical @blur @color;
}
.inset(@dims:1px 1px 1px, @color:#fff) {
    box-shadow: @dims @color; // Opera, FFX4
    -moz-box-shadow:@dims @color; // FFX3.5
    -webkit-box-shadow:@dims @color; // Safari/Chrome/WebKit
}

.box-size() {
    -moz-box-sizing: border-box; -webkit-box-sizing: border-box; box-sizing: border-box;
}

.rotate(@deg: 45deg) {
    -moz-transform: rotate(@deg);
        -webkit-transform: rotate(@deg);
        -o-transform: rotate(@deg);
        -ms-transform: rotate(@deg);
        transform: rotate(@deg);
}

.transition (@time: .4s, @range: all, @ease: ease-in) {
  -moz-transition: @range @time @ease;
  -webkit-transition: @range @time @ease;
  -o-transition: @range @time @ease;
	transition: @range @time @ease;
}

.transition-delay (@time: .2s) {
	-webkit-transition-delay: @time;  
	-moz-transition-delay: @time;  
	-o-transition-delay: @time;  
	-transition-delay: @time; 
}

.rounded(@radius: 5px) {
	-moz-border-radius:@radius; // FFX1-3.5
	-webkit-border-radius:@radius; // Safari 3/Chrome 0.2
  border-radius: @radius;
}

body { 
  width: 100%; 
  background: url(http://subtlepatterns.subtlepatterns.netdna-cdn.com/patterns/grid.png) repeat #fefefe;
}

.wrap {
  width: 500px;
  margin: 100px auto;
  
  
  /* button stuffs */
  button {
    font-family: 'Archivo Narrow', sans-serif; 
    font-weight: bold; 
    font-size: 36px; 
    .textinset; 
    color: #333;
    display: block; 
    padding: 10px 30px; 
    margin: 0 auto; 
    min-width: 200px; 
    .box-size; 
    position: relative;
    z-index: 1;
    .inset;
    .default;
    
    /* span stuffs (tooltip) */
    span {
      .box-size;
      position: absolute;
      padding: 10px;
      width: 190px;
      .rounded;
      .textinset(0,0,0);
      font-weight: normal;
      color: rgba(255,255,255,0);
      opacity: 0;
      font-size: 16px;
      .transition;
      .transition-delay;
      z-index: 1;
      .nub{
        display: block; 
        width: 10px; 
        height: 10px; 
        padding: 5px;
        border-style: solid; 
        border-color: transparent; 
        background: rgba(255,255,255,.8);
        .box-size; 
        position: absolute; 
        left: 42%;
        .rotate;
        z-index: 0;
        .transition;
        .transition-delay;
        .rounded(0px);
      }
      &.top {
        top: 0px;
        left: 4px;
        .nub {
          left: 20%; 
          bottom: -8px;
        }
      }
      &.bottom {
        bottom: 0px;
        left: 4px;
        .nub {
          left: 20%; 
          top: -8px;
        }
      }
      &.left {
        top: 0px;
        left: 4px;
        .nub {
          left: 180px; 
          top: 20px;
        }
      }
      &.right {
        top: 0px;
        right: 4px;
        .nub {
          left: -8px; 
          top: 20px;
          margin: 0 0 0 0;
          .transition;
        }
      }
    }
    &:hover {
      span {
        background: @tipcolor;
        height: auto;
        color: #fff;
        font-size: 16px;
        opacity: 100;
        .transition;
        .transition-delay;
        .nub {
          .transition;
          .transition-delay;
        }
        &.bottom {
          bottom: -60px;
        }
        &.left {
          left: -193px;
        }
        &.right {
          right: -193px;
        }
        &.top {
          top: -60px;
        }
      }
    }
  }
}