Inventory, unlike monitoring, refers specifically to a point-in-time. When ecologists, range management professionals, or others talk about inventory, they are referring to a point-in-time measurement to assess the condition of a resource.
For example, ecologists may inventory the population of a rare plant. Or, range managers may inventory the condition of resources on a ranch. Inventorying is the systematic acquisition and analysis of information needed to describe, characterize, or quantify rangeland resources. Inventories can be used for mapping the extent and location of various rangeland resources such soils and vegetation. Inventory data may also be interpreted to assess ecological status of these resources. (by Rachel Frost, Montana State University)
The actual type of data collected during inventory may not actually differ from the type of data collected during monitoring, but the big difference is that monitoring is repeated. Often, inventory data may be a baseline that informs future monitoring.
Elzinga, C.L., et al. (1998). Measuring & Monitoring Plant Populations.