Human Beings: a topic we all know well

We are all human beings. We have all had years of experience being human beings. We have, further, been observing, interacting with and generally learning about other human beings: family, friends, classmates, colleagues, and so on. I think it's safe to say that we are all very familiar with the topic of human beings.

To narrow the topic a little, I think we have all previously heard, and perhaps repeated ourselves, the idea that all human beings are equal. This equality is common in political and moral discussions - it's a foundation of democratic countries; we use it argue against the injustice of laws allowing slavery and the immorality of laws that treat women as inferior to men; and human equality is a main support for the idea that all human beings have dignity, deserve respect and must be treated in certain ways by governments, legal systems, society, other human beings and ourselves.

But this idea that all human beings is not clear or simple:

• What does it mean to say that human beings are equal? 
• In what way are we all equal? 
• What do we human beings all have in common that makes us equal? 
• Is it even true? Are all human beings really equal in any way that is politically, socially or morally important? 

Although this topic is one that I am sure is familiar to all of us, I am equally sure that the ideas I've raised in the questions are very complex. I'm sorry for starting off with something so difficult, but it will help our coming discussion to start thinking about these sorts of issues now, so comment!