These terms get used interchangeably sometimes, but they describe different scopes of work with different price points.
Brush clearing
Brush clearing removes undergrowth, saplings and brush while generally leaving mature trees standing. It's the right call for reclaiming overgrown pasture, cleaning up fence lines, or improving a property's usability without changing its overall character.
Full land clearing
Full land clearing removes trees as well as brush, typically to prepare a site for construction, a pond, or a significant change in land use like converting wooded acreage to open pasture. It's more involved and costs more per acre because trees — especially mature ones — take more time and equipment to remove than brush.
How to decide
Ask what the end goal for the property actually is. If you want the land opened up and usable but don't need every tree gone, brush clearing likely gets you there for less. If you're preparing a specific footprint for construction or need trees out for a defined project, full clearing is probably necessary for at least that portion of the property.