The boolean operators do what you expect with boolean values. For values that are not boolean, everything evaluates to true, except nil, 0, "" (an empty string) and [] (an empty list), which evaluate to false.
The boolean operators are:
&
| boolean and, evaluates to true if both operands are true |
|
| boolean or, evaluates to true if one of the operands is true |
!
| boolean not, evaluates to true if its operand is false |
The logical operators only evaluate their operands when needed: & doesn't evaluate the second operand when the first one is false, | doesn't evaluate the second operand when the first one is true.