Hi,

A trigger means programming in APEX (a java look-a-like) and you can do things with a trigger that no Workflow actions can do.

Worksflow rules however, allow you to do a lot of useful thinsg with zero programming, like updating fields and sending out emails based on rules. Very neat.
Workflow is used for Update,Send Email, Craeting Task based on the criteria you specified. Previously you can use workflow when you want something to be happening on the same object but with Spring 12 you can use this with cross object like formula field. You cannot update a lookup field using workflow similarly you will find more . So by using Cross workflow you can work on to different object but there must be a relation between them.
On the other hand you can write trigger on a particular object and so something on other object even if there is no relation between these object. For example suppose you want whenever a Account is inserted all the Custom Object Record should be updated while you don’t have any relation between account and custom object. Try the below as reference:
Trigger accountTest on account(after insert)
{
List<customObject__c> cus=[select id,name from customObject__c limit 1000];
Update cus;
}
You will know more when you will work on this.

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