<ul>
<li>What exactly is a learning curve?</li>
<li>And why is it wrong to use the term "steep learning curve" for something which has high entry barriers and takes quite some time to get into?</li>
</ul>

<p>As to the why-ness of this question:</p>

<ul>
<li>The terms are used often and inconsistently on Stack Overflow</li>
<li>I myself have been confused by it</li>
<li>Mostly the newbies are confronted with these terms when they for example ask questions like "what's the best php development framework"</li>
</ul>

from Widipedia:
The term learning curve refers to the
  graphical relation between the amount
  of learning and the time it takes to
  learn.
The term "steep learning curve" is often
wrongly
used for things which need some time to wrap ones mind around. Also here on Stack Overflow I've seen it used wrongly many times and hence this Question and my own answer to it.
In fact a steep learning curve is given, when it is relatively easy to start of with a new skill/technique/...
it means that the relationship between "learning progress (y)" and "time invested (x)" is greater than 1.