Difference between force stop and kill process
Greenify and Task killer kill your apps, but Greenify also disable the receiver in the greenified apps. This way the apps won't be able to start again stealthily. The pro version can act as a GCM proxy, in which GCM messages are rerouted to Greenify and it can wake a greenified apps, giving you the best of both world (no RAM and CPU usage, yet still able respon to push message) for messaging and email apps.