Here’s how Placemark relates to a few other things in the world of geospatial tools!
Placemark uses a lot of pieces from Mapbox - technology like Mapbox GL JS, the Mapbox geocoding service, and Mapbox's map tiles for the underlying base map. The closest product Mapbox has to Placemark is the Dataset editor in Mapbox Studio, which the author of Placemark contributed to. Placemark focuses entirely on creating, editing, and publishing data, while Mapbox Studio is mainly focused on styling and designing maps.
The author of Placemark created geojson.io. Placemark aims to be the same, but better.
OpenStreetMap provides the map data that you see in Mapbox's base maps, as well as lots of other places on the internet. OpenStreetMap (OSM) allows map editing through the iD editor (which the creator of Placemark also worked on in its early stages), but the OpenStreetMap map is public, shared between everyone, and strictly refers to certain kinds of data like streets and businesses.
Placemark can support any type of data and is intended to be used for private data as well.
QGIS is a powerful desktop tool for editing maps. It's open source and great, but can be a little intimidating to use. Placemark is meant to be simpler and easier to get started with. You might want to use both QGIS and Placemark if you need more advanced analysis.
ArcGIS is similar to QGIS, but is an older proprietary product. It tends to be more expensive, but is powerful. Unfortunately, ArcGIS promotes and supports proprietary formats.