There was a recent topic reagrding this:
In short you can use both.
- A form is usually to collect emails on a website, a contact form.
- A Campaign form is when you want to start a campaign right away after submission.
- To start a campaign with a segment happens, when you want to communicate with your current list. Maybe you want to filter the segment based on their visiting habits or custom field values.
I hope this helps.
Joey