Transformation Rules for Functions
"Values for Symbols" discussed how you can use transformation rules of the form x->value to replace symbols by values. The notion of transformation rules in the Wolfram System is, however, quite general. You can set up transformation rules not only for symbols, but for any Wolfram System expression.
Probably the most powerful aspect of transformation rules in the Wolfram System is that they can involve not only literal expressions, but also patterns. A pattern is an expression such as f[t_] which contains a blank (underscore). The blank can stand for any expression. Thus, a transformation rule for f[t_] specifies how the function f with any argument should be transformed. Notice that, in contrast, a transformation rule for f[x] without a blank, specifies only how the literal expression f[x] should be transformed, and does not, for example, say anything about the transformation of f[y].
"Patterns" and "Transformation Rules and Definitions" explain in detail how to set up patterns and transformation rules for any kind of expression. Suffice it to say here that in the Wolfram System all expressions have a definite symbolic structure; transformation rules allow you to transform parts of that structure.