An introduction to using attributes and local variables in Houdini. Covers
Different types of attribute : vertex, point, primitive and detail.
When attributes and variables can be used, and when not.
Vector vs 3 value float attributes.
Converting between point and detail attributes etc.
Note that with Houdini 10 there is an alternative way of referring to attributes in expressions. You can for example use @P.x instead of $TX. Confusingly the old variable names apply in some places, while they do not in others. If you've created an attribute and want to access it in an expression try using @name, where name is the name of your attribute. If this doesn't work try $NAME. Note that the attributecreate SOP no longer creates variable names (such as $NAME) by default. You need to enable the option to create local variables.