Something that really distinguishes geospatial data from images or drawings is the prominence of properties. Map features aren't just geometries, like a polygon in the shape of Manhattan, but also contain data that describes what that shape is, in the world. Properties are usually stored as pairs between a "key" and a "value", like key=name, value=Manhattan. You can think of this like rows in a spreadsheet - each column is a key, and each row has a value for each key.
Another distinguishing fact about properties in geospatial data is that they're freeform. While some mapping tools will limit you to input a "name" and "description", in the general sense any feature on a map can have any set of properties - any names and any values. Placemark embraces this fact and is not opinionated about properties names and values. One of your datasets might refer to things with a "name" property, another might use "title" or "id". Any of the following will work.
Properties in geospatial data have types. A given property might be a number, some text, as boolean value like true or false, or even a complex array or object for software to use. Placemark supports all property types that are valid in GeoJSON, which is one of the most permissive and flexibly-typed formats.
While properties can be compared to rows in a table, they're more powerful than that. In a given dataset, different features can have different sets of properties. If one feature has a property like "height=42", other features can have a height property, or omit that property entirely. In this way, properties are more freeform that tables.
Placemark supports editing rich text properties, which are represented as HTML in code but are edited with a friendly non-coding editor. You can upload images that'll be automatically hosted and shown in the rich text, as well as add styles like bold, link, strikethrough, and headings.
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