The default login page for any wordpress login page is wp-login.php. It is easy for some one to get into your login page and can access your site. By default username is admin people wont change generally and I explained here how to change the admin username.
By some trail and error or by some brute force approach some bots can easily crack your password. So as to get out of incidents changing the login URL using Stealth Login will help.
This plug-in can change the login, logout and administrator URL so that the default http://yourdomain.com/wp-login.php can be changed to http://yourdomain.com/login ( say login is my new changed one). If your blog became popular the chances of breaking your passwords will be more.