[texhax] help with identifying some macros

[P. R. Stanley]

Hi folks
\strut, \vtop and lblot. I'd be grateful for a brief description of 
each, more precisely, the general effect on the presentation.

Here's an example of two of them in use taken from The Z Notation: a 
Reference Manual by Michael Spivey.

$birthday$:
\[
	known = \{\,{\rm John, Mike, Susan}\,\} \\
\also
	birthday = \{\,\vtop{\halign{\strut#\hfil&${}\mapsto{}$#\hfil\cr
			John&  25--Mar,\cr
			Mike&  20--Dec,\cr
			Susan& 20--Dec\,\}.\cr}}
\]
Truth be told I've seldom come across anything so cryptic. For 
example, what does the "#" signify?
Any help would be most appreciated.
Thanks in advance
Paul