What Does @ Mean?

[Ron Tarrant]

This is frustrating and makes me feel like a complete newb. 
Worse, it's impossible to search for. Ever try Googling a single 
character?

The D documentation also doesn't seem to explain the meaning of 
this or any other token. Sure, most of them are obvious, but this 
one eludes me. All I can find is this: 
https://dlang.org/spec/lex.html#tokens

Would someone please tell me what an at sign (@) means when it's 
used like this:

bool isLeaf() @property
{
    return children.length == 0;
}

In fact, I have no idea what @ means anywhere other than in an 
email address. Is this a common thing in contemporary languages?

And while I'm asking, does an underscore have special meaning 
when used either at the beginning or end of a variable name?

How about a double underscore?

I know underscores are sort of left over from C, but do these 
characters have special meaning in D or are they simply a 
convention... like T in a template definition?