<button>모달창</button>

<div class="modal">
  <div class="modal_content" 
       title="클릭하면 창이 닫힙니다.">
    여기에 모달창 내용을 적어줍니다.<br>
    이미지여도 좋고 글이어도 좋습니다.
  </div>
</div>

button{ position:absolute; top:50%; left:50%;
  width:120px; height:30px; margin-top:-15px; margin-left:-60px;
  line-height:15px; cursor:pointer;
}

.modal{ 
  position:absolute; width:100%; height:100%; background: rgba(0,0,0,0.8); top:0; left:0; display:none;
}

.modal_content{
  width:400px; height:200px;
  background:#fff; border-radius:10px;
  position:relative; top:50%; left:50%;
  margin-top:-100px; margin-left:-200px;
  text-align:center;
  box-sizing:border-box; padding:74px 0;
  line-height:23px; cursor:pointer;
}