Show content when hovering over DIV

Question 1

Assume you have the following markup:

<div id="parent">
    Some content
    <div id="hover-content">
        Only show this when hovering parent
    </div>
</div>

The CSS:

#hover-content {
    display:none;
}
#parent:hover #hover-content {
    display:block;
}

This should do the trick.

Not sure how you'd do it with transitions, but you'd have to put the same selector at least.

Example

Question 2

If, per say, you have this context :

<div class="flowingdown">
    <div class="something-inside">
        something-inside
    </div>
    <div class="something-inside-but-hidden">
        something-inside-but-hidden
    </div>
</div>

CSS

.something-inside-but-hidden {display:none}
.flowingdown:hover .something-inside-but-hidden {display:block}

Working example jsFiddled here

Question 3

Yes, it is possible.

But wrap your .flowingdown within a div. Inorder to show the entire content,

<div style="width: 200px; height:300px;">
    <div class="flowingdown"></div>
</div>

CSS:

.flowingdown {
    width:1045px;
    background-image:url(http://i.imgur.com/hHUa9Ty.jpg);
    height:50px;
    float:left;
    margin-top:2px;
    margin-bottom:2px;
    border-radius:0 0 55px 55px;
    transition: height 1s;
    -webkit-transition: height 1s;
}
.flowingdown:hover {
    height:100%; //Inorder to show the entire content within the parent.
}

Check this JSFiddle

Question 4

I assume you are trying to hide the bottom half of the background-image, but I just tested this exact code and it worked fine:

<html>
<head>
<style>

.flowingdown {
    width:1045px;
    background-image:url(../Pictures/C_2560x1400.jpg);
    height:50px;
    float:left;
    margin-top:2px;
    margin-bottom:2px;
    border-radius:0 0  55px 55px ;
    transition: height 1s;
    -webkit-transition: height 1s;


}

.flowingdown:hover {
    height:100px;
}
</style>
</head>
<body>

<div class="flowingdown">

</div>

</body>

That's the same exact code you used, except I used an image of my own.

If what you want is to change the background image on hover, your CSS should be:

.flowingdown:hover {
    height:100px;
    background-image:url(path.jpg);
}

Let me know if I misunderstood your question.