The nodes have consistent behaviors as much as possible, so here are a few notes written once here instead of being repeated all over the place:
You can easily access the reference documentation of any node: select it, open the N-panel, click on the Doc button. Some nodes don't have a reference page yet, it's a work in progress.
When a node creates Blender objects, it will be in the current scene, and currently active collection. (This is not true for the NEW nodes introduced in 1.8, check the main page, what's new section)
When a node offers the option to delete what it creates, that means creating will not delete or overwrite the previous items. Also datablocks that have several users (eg. a mesh that is used by at least two objects), will be silently ignored and not deleted.
When a node asks for a name prefix for what it will create (objects, meshes datablocks etc…), try to make that prefix unique to that node, it will avoid potential errors. Especially if the node has a button to delete what it creates, it will delete the items based on their name prefix.
If a button on a node says “Create / update” it means that the item will be created if it doesn’t exist yet, or updated otherwise. When the data is updated, it means all the data inside (for example all verts and faces for a mesh, or all the pixels color for an image) will be overwritten, and any manual change will be lost.
If a node has a random seed, that means that, even though the operation it performs is based on randomness, it will always give the same result again and again as long as you do not change any setting. Changing any setting will give a totally new random result. If you simply want the same kind of result with the same settings, but a different random one, then change the seed. The exact value of a seed doesn’t matter, but it can be useful to note it down if you find a particular value that gives a result you like, while you’re experimenting.