The equals symbol is used in math to assert that two expressions have the same value.
The not equal symbol is used in math to indicate that two expressions are not equal to each other.
The approximately symbol is used in math to indicate that two expressions are approximately equal to each other.
The equivalent symbol is used in modular arithmetic to express that two numbers are congruent modulo some number N. The symbol is also used to express an identity.
The congruent symbol is used in geometry to state that two shapes are identical to each other.
Two stacked horizontal lines respresents the equals symbol in mathematics. The two expressions on either side are equal, or the same, when the equal sign is placed in between them.