1.Action is a thing specific to QTP while functions are a 
generic thing which is a feature of VB Scripting. 

2.Action have a object repository, Data table etc 
associated with it while a function can't. 

3.A function is just lines of code with some/none 
parameters and a single return value while an action can 
have more than one output parameters. 

4.We can write a function in an Action...But in Function we 
can't..

5. We can't call an Action in a function, But we can call a 
function in an Action.

we can pass values in functions ,but not in actions

functions are faster than actiions