Email has been around since the invention of the internet and unsurprisingly has the highest ROI out of every marketing channel with the exception of search. It's also one of the most flexible allowing you to set the rules of when, who, where and how.
The first, most frequent and easiest type of content is text. Like cash, it's accepted everywhere and everyone can read it (including blind people, using a screen reader). The downside is that it's crap at conveying emotion and you have to be a confident writer.
The next option is images, thanks to the rise of the smartphone. Prior to the standard internet interfaces, email creators had to create code for several different types of display platform, with Microsoft Outlook being the worst culprit. Smartphones standardized this, allowing colourful and creative emails to break loose. Thanks to the safe-sender list, you were also more likely to display your images. The downside, now? Big images eat up data on phones and you still need to pay for image production or creation. You can use free stock images, but will a generic look sell your brand?
Finally, there's the video option. This is cheap, underrated and really spectacular when done right. Not every email should contain a video, but a small personal message at the right time can triple your engagement (and sales!). They actually work best for service-based providers who are basically selling themselves. Answer a question, display a solution, give a tip. You need to keep the video short and sweet (no more than three minutes), with a back -p image and link for inboxes that do not automatically display the video screen. You also need to add little intro text, so your audience knows it's a legitimate message. You can create it and send it in the time you have taken to read this blog post, using just your smartphone and a place with good lighting.
Have fun!