Salesforce.com Apex Classes and Apex Triggers

Apex Classes:
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You can write sf.com Apex code in Apex classes. Using this Apex classes you can call your own application and pass data from sf.com.

Apex classes can be called from Apex Triggers. Apex classes having logging mechanism. You can create apex classes with any salesforce.com Versions.

Setup -> App Setup -> Develop -> Apex Classes

Apex Triggers:
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You can create your own Apex triggers, and the trigger can be called when you create / update accounts, contacts, opportunity etc..

From Apex trigger you can call your Apex Classes and Apex class will sync information to your application.

Setup -> App Setup -> Develop -> Apex Triggers

This apex classes and Apex Triggers is specific to the sf.com credential, in which credential you deployed.

Debug sf.com System Logs:
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Login into sf.com. In the top right corner you can see System Log next to setup. Click System Log (Its for debugging and executing and testing your classes and triggers).
1. Click show filter settings (Make sure Apex Code : DEBUG level for debugging the errors.)

Type the Apex trigger / class code in Execute Apex and click 'Execute' button. See the errors in errors log list.