your question depends a lot on what you want to do ..
But in general , when a plugin is loaded or executed, the functions are being “registered” in PHP server and are available for all to use (depending of course HOW you write them) …
So for example, to detect if a plugin is enabled or installed, the other plugin could have
if (function_exists('NameOfFunctionFromPlugin1')
there is also a wordpress function to check plugin activation
<?php is_plugin_active($plugin) ?>
http://codex.wordpress.org/Function_Reference/is_plugin_active
as for “transmitting Info” : Because the functions are available, you could use them. so for example, a value that is returned from a certain function in one plugin, can be evoked by the other , calling this function , and getting the value returned for use .