Is there any difference between these two version of MySQL - 5.1.73-community-log Vs 5.1.73-community I have installed MySQL 5.1.73 in Oracle LINUX 5 server. Just after RPM installation, I checked system variables and found that version is 5.1.73-community.
When I copid  /usr/share/doc/MySQL-server-community-5.1.73/my-medium.cnf to /etc/my.cnf
I got 5.1.73-community-log as value of version variable.
I am curious to know about what is the difference between 
5.1.73-community and 5.1.73-community-log
They are the same software. The -log suffix is simply appended to the version string at runtime.
Here's an explanation (nearly from the last century), by long-time MySQL author Paul DuBois:


I want to know what the -log means and how it got in there.

-log is added if you start the server with logging turned on.
-debug is added if you're running a debugging version of the server with debugging on.
-demo is added if the server is running in demo mode

I verified this in the source, in sql/mysqld.cc, function set_server_version():
. . .
if (opt_log || opt_slow_log || opt_bin_log)
  strmov(end, &quot;-log&quot;);                        // This may slow down system
. . .

I think the meaning of that comment is not that appending four extra characters to the version slows down the system, but that logging may slow down the system, so it's worth noting to the DBA that logging is enabled.