Before diving into the differences between Workflow Rules and Process Builders, I do want to call out that in some Trailheads from Salesforce, there have been callouts to moving away from Workflow rules entirely. Be that as it may, Workflows are still up and running and in some cases can be simple, faster, and easier to apply than a Process Builder. However, please do keep this in mind if you are starting off in a new org, as it may be better, in that case, to solely use Process Builders.

What is a Workflow Rule?

A Workflow Rule is a point and click version of an If/Then statement, with a limited number of If statements are accepted and a limited number of Then actions. Workflow Rules function on the record that is triggering the statement or in some cases, the child of the record that is triggering the statement (you cannot go upstream in a Workflow rule).

There are two ways to create the If part of the statement (otherwise known as the Rule Criteria):

1. Use the Wizard Salesforce provides you:

2. Create your own custom formula

The formula method can be useful if you were trying to create a formula field but you ran into a character limit on the field, you can then use the Update Record action instead, setting the Rule Criteria as the desired formula - more on the actions in a moment.

Once you have set the If statement, then you can move on to what happens when the If statement is evaluated to True. The Then statement is referred to as the Rule Action.

Sending an Outbound Message is the only item in this list that can be solely done with a Workflow rule. At the time of writing, there is no other method to achieve this.

Workflow rules are still most commonly used for generating a task or sending out an email alert. As mentioned at the beginning of the article, Salesforce is making a push towards Process Builder and away from Workflow Rules, and updating records is happening more and more in Process Builders (continue reading to learn why).

The reason you may pick a Workflow rule over Process Builder, besides for the Outbound Message, is if your logic is simple or does not matter the order that is hit in comparison to other Workflow rules(see this article on Order of Operations for more information). Beyond that, at this point Process Builders win the race. With that being said, let's move on Process Builders.

What is a Process Builder?

A Process Builder is a simplified flow that allows for multiple If/Then statements and can be kicked off in multiple ways (a big distinguisher from Workflow Rules). A Process Builder can start from a change to a record (create/update), from a Platform Event Message, or from another Process Builder or Flow.

Generating the If part of the statement, otherwise called setting the criteria, is similar to the Workflow rule in that you can use the Wizard element:

Or you can create your own formula:

An additional benefit over the Workflow rule criteria setup is that in Process Builder there are more fields accessible to you to derive the criteria, as well as some global statements, like ISNULL(). This is important because in a Process Builder you can validate a condition based on a related record, but to do so does require also checking that the field is not null, otherwise you are likely to get a flow error.

As you can see from the list above there are so many more options available in Process Builders than there are in Workflow rules as to what actions can be taken. When creating the Process Builder you can opt to do one or more of these items together from one criteria node, or alternatively, you can have it continue from the first step and then proceed to check new criteria and do additional actions. 

What is great about all of this functionality, and likely why Salesforce is pushing forward with Process Builder, is that you can control the order the actions happen in and how many actions per criteria. The idea is to have a single Process Builder for each Object that you have and then construct a chain of Process Builders or Flows from the first one - as you can see from the list you are able to both start from a Process Builder as well call out to another Process Builder giving you a great amount of flexibility.

All of this flexibility of the Process Builder also helps admins to be able to have more options in a click and not code as the Process Builder can be constructed very similar to a trigger, with the ability to say take action when something is new or something has changed.

As by now, I am sure you can see that there are very few reasons to go forward with a Workflow rule over a Process Builder. However, for reference here is a comparison of the actions you can take with each as well as a Pro/Con list for the different options.