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
there is also a wordpress function to check plugin activation
<?php is_plugin_active($plugin) ?>
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 .