In Houdini there are many different types of folders. These folders are associated with a network type. It very import to understand what these folders are called, and how they work.

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SOP: Surface Level Operator. This operator includes any geometry that is in an object folder. You use this operator to modify, edit, and build your geometry.

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DOP: Dynamic Operator. This operator is where you build your simulation. This includes any solvers, or simulation nodes you create.

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Object: Object Type Node. A simple example of this node is a regular geometry node that contains your FBX, OBJ, or any type of geometry you would like to edit.

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 VEX: Vector Expression Language.  The standard Houdini code to write shaders.

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SHOP: Shading Operator.  These contain your materials for your geometry. They represent your shaders that you can apply. Just like any shader, you modify some of them, others you are unable to.

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VOP: Vector Operator. Inside this node, you have the freedom to modify almost anything. From shaders, geo, volumes, VDBs, points, etc. 

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CVEX: Context Agnostic Vector Expression Language. This is a new add on to VEX in Houdini. It is now treated as Houdini's general language. 

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COP: Composite Operator. Here, you can use these nodes to composite image sequences, and images behind your created VFX, or combine FX over them.

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ROP: Render Operator. Your render nodes! Mantra is an example of this operating type. This operator, contains, receives, outputs, and complies the information/simulation you would like to render.

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CHOP: Channel Operator. This operator is used for raw channel data. Raw channel data would contain audio, or motion data.

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Houdini Help:

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If you are using a version of Houdini 17 or higher, you'll notice that some nodes are missing. Such as the fluid source SOP node, xform, and some others.

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In the case of Xform, it is now called the Transform SOP. In the case of finding the new fluid source node...well that's a bit harder.

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To find the old fluid source or older nodes, you'll need to open up the Hscript editor. And use this script:

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• opunhide Sop fluidsource  

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In this case, the opunhide will be used to unhide the node you'll be looking for. The Sop part of the equation is the folder/network type of the node you are looking for. If your node is a Dop for example, you would exchange Sop for Dop. Then the fluidsource in this case, is the node name. You'll replace this if you are looking for a different node.