# Specify a language in Relevant Tags to narrow down question scope. I'm proposing an answer in Python because I see in your profile you are learning it.
# The `!=` operator is used to compare values of its operands. It returns a boolean state, True when both operands' value equals, False otherwise.
a, b = 1, 2
print("a == b ->", a == b)
# `==` operator test for equality
print("a != b ->", a != b)
# `!=` operator test for inequality
# The `not` operator returns an inverted logical expression.
# It returns True when the operand was False; or return False when the operand was True.
# It only accepts one operand to work.
# Basically, this is a logical operator, so its operand ideally should be something than can be evaluated as boolean expression.
young = True
print("young ->", young, "not young ->", not young)
if not young:
print("You are old")
print("You are young")