css syntax : what is + ?

[html20009876]

I saw css code somewhere with '+', I can't remember exact format.
i.e.

.... #first + #second ...
{
   ...
}

What does the '+' mean in css?
How many ways to use it?

i.e. (maybe incorrect in format)

#first + #second > div
{  
   ...
}

or

#start > #first + #second
{  
   ...
}

or more? or else?

Thanks.
.