ButtonBoxOptions -> {opt_1 -> val_1, opt_2 -> val_2, ...} is an option for cells that specifies settings for buttons within the cell.

FrontEndEventActions is an option for the notebook front end that gives a list of actions to perform when specified user-interface events occur.

Unitize[x] gives 0 when x is zero, and 1 when x has any other numerical value.

$MessageList is a global variable that gives a list of the names of messages generated during the evaluation of the current input line.

$PreRead is a global variable whose value, if set, is applied to the text or box form of every input expression before it is fed to Mathematica.

This gives a result for the integral of x^n that is valid for almost all values of n. For the special case of x^-1, however, the correct result is different. The overall goal ...

The simplest way to mix text and formulas in a Mathematica notebook is to put each kind of material in a separate cell. Sometimes, however, you may want to embed a formula ...

Just as Mathematica allows you to define how expressions should be evaluated, so also it allows you to define how expressions should be formatted for output. The basic idea ...

"Defining Functions" discusses how you can define functions in Mathematica. In a typical case, you would type in f[x_]=x^2 to define a function f. (Actually, the definitions ...

Converting between symbols and their names. Here is the symbol x. Its name is a string.