Email newsletters are often different from a specific campaign in that they run indefinitely. On the one hand, lead nurturing campaigns are usually just a series of messages, about four to six emails total. The content in these emails is specific to the kind of campaign you're running.
Newsletters go indefinitely. They a channel for regular communication between you and your leads. They're not sales focused. Rather, they act as a service, where you continually deliver helpful content that leads your recipients to a solution.
Sometimes, newsletters can be centered on product updates, curated blog content, or just some fun, entertaining content that fits your audience. The most important element of your newsletter is relevance. When you're creating your newsletters, always consider how your content is helping your audience.