Adobe Spark is free, but you have to create an Adobe ID and be logged in to use it.
Spark on the Web unifies all your projects, regardless of format.
Unlike the iOS apps, which exist individually, the Web app has a central starting point for any project.
Before you start, it shows you an outline of the process.
All of these are actually the same; they just have custom text prompts which try to give you some guidance. For instance, the Hero's Journey template has five slides; the first is Setup, with the prompt "Tell us about the hero and their world before the quest begins." In this respect, Spark Video is the least smart of the apps.
You can create videos using three types of elements: photos, text or (in my opinion) select from a set of rather cheesy iconic graphics. Oddly enough, you can't add videos in Video.
Video comes with a variety of themes that you can't really deviate from.
The app has a reasonable selection of usable fonts, and as you resize the text box it scales and wraps the text automatically.
You can change the text-box shape; text and accent colors; typeface; increase or decrease line spacing; alignment; and the transparency of the shape. The style suggestions control cycles through all the variations built into the themes.
You can choose from quite a few different background shapes for text.
There are palettes that you can cycle through as well as individual colors you can choose for text and shapes.
You can scale and rotate images as well as apply filters designed to work with the preset themes.
Spark is targeted at small businesses, nonprofits and other working folks who don't create graphics professionally, so I guess Adobe thinks that spamming people with a post watermarked #AdobeSpark looks professional.
Unlike Post, you're restricted to the formatting defined by each template.
I wish there were a larger selection of templates and the ability to remove effects filters like you can do in Spark Post.
You can embed videos from YouTube, Vimeo or ones created by Spark Video (which are hosted by Adobe).
You can add picture-in-picture scrolling boxes with photos, captions, buttons and video embeds.
Page can also automatically create photo grids.