Logs are records of all kinds of events. You can be as granular as you want: the more information you're recording, the more you can look back on and learn from in the future.
There are a number of different type of logs in farmOS - each with it's own purpose and set of fields. Some of the general log types are described below, but there are also other log types that pertain to specific asset types.
Activities are a sort of catch-all, or default, log type, which can be used for general planning and recordkeeping of activities that don't fit any of the other, more specific, log types.
Movements are used to record the movement of a farm asset from one area to another. Movement logs are the primary way that an object's location is determined.
Observations are used to record any kind of passive observation on the farm. For example, seeing that a planting has germinated is an observation. This is a very flexible log type that can be used for a lot of different things. It comes with it's own "Observation Type" vocabulary for defining your own custom categorizations.