Use the 
Boolean tool on the Model tab to combine, intersect, or subtract the surfaces of two objects.
Use the 
Combine tool on the Modify tab to merge the geometry inside two Geometry container objects into one object.
See the /model/help/shelf/index.html for the Modify tab, Model tab, Polygon tab, and Deform tab for more modeling tools.
Describes how Houdini represents geometry using details, primitives, points, vertices, and attributes.
In Houdini you can not only edit the parameters of a surface node, you can reselect the geometry the operation applies to.
Building terrain with height fields
How to use Houdini’s height field tools to generate realistic terrain.
Realistic terrain with heightfields
A heuristic workflow using the heightfield tools, based on experience generating realistic-looking terrain.
How to break different types of materials.
How to create 2D and 3D cloud patterns using the Sky Field node.
The appearance editor mode of the data tree pane lets you edit various controls for object viewport/rendered appearance in one place.
You can mark a chain of compilable SOPs as a compiled block which can execute much faster in parallel.
Programmatic geometry with verbs
Many geometry (SOP) nodes allow you to use them in Python scripts to generate geometry programmatically.
Speed vs. reproducibility in Math Kernel Library
Some nodes use a library that gives a speed boost for floating-point operations but can make the output vary between different computers. You can control how much the library tries to guarantee reproducible results.