//cube is directly inside the container:
#container:hover > #cube { background-color: yellow; }

//cube is next to (after containers closing tag) the container:
#container:hover + #cube { background-color: yellow; }

//If the cube is somewhere inside the container:
#container:hover #cube { background-color: yellow; }

//If the cube is a sibling of the container:
#container:hover ~ #cube { background-color: yellow; }

div {
    display: none;
}
    
a:hover + div {
    display: block;
}

.showme {
  display: none;
}

.showhim:hover .showme {
  display: block;
}


<div class="showhim">HOVER ME
  <div class="showme">hai</div>
</div>

#a:hover + #b {
    background: #ccc
}

<div id="a">Div A</div>
<div id="b">Div B</div>