The difference between Backup and Spare

When used as nouns, backup means a reserve or substitute, whereas spare means the act of sparing.

When used as adjectives, backup means standby, reserve or extra, whereas spare means scanty.

Spare is also verb with the meaning: to desist.

check bellow for the other definitions of Backup and Spare

Backup as a noun:

A reserve or substitute.

Examples:

"If the goalkeeper is injured, we have a backup."
Backup as a noun (computing):

A copy of a file or record, stored separately from the original, that can be used to recover the original if it is destroyed or damaged.

Examples:

"After the power failure, we had to restore the database from backup."
Backup as a noun:

An accumulation of material caused by a (partial) obstruction or (complete) blockage of the flow or movement of the material, or an accumulation of material that causes an overflow due to the flow being greater than the maximum possible flow.

Examples:

"The accident caused a mile-long backup on the highway."

"The blockage caused a backup in the plumbing."
Backup as a noun (law enforcement):

reinforcements

Examples:

"He's got a gun! You'd better send for backup."

Backup as an adjective:

Standby, reserve or extra.

Examples:

"I am only a backup player."
Backup as an adjective (computing):

That is intended as a backup.

Examples:

"Make a backup copy of that file."

Spare as an adjective:

Scanty; not abundant or plentiful.

Examples:

"a spare diet"
Spare as an adjective:

Sparing; frugal; parsimonious; chary.
Spare as an adjective:

Being over and above what is necessary, or what must be used or reserved; not wanted, or not used; superfluous.

Examples:

"I have no spare time."
Spare as an adjective:

Held in reserve, to be used in an emergency.

Examples:

"a spare anchor; a spare bed or room"
Spare as an adjective:

Lean; wanting flesh; meager; thin; gaunt.
Spare as an adjective (UK, informal):

Mad: angry or crazy.

Examples:

"When he found out that someone had broken the window, he went spare."

"The poor girl is going spare, stuck in the house all day with the kids like that."
Spare as an adjective (obsolete, UK, dialect):

Slow.

Examples:

"rfquotek Grose"

Spare as a noun:

The act of sparing; moderation; restraint.
Spare as a noun:

Parsimony; frugal use.
Spare as a noun:

An opening in a petticoat or gown; a placket.
Spare as a noun:

That which has not been used or expended.
Spare as a noun:

A spare part, especially a spare tire.
Spare as a noun:

A superfluous or second-best person, specially (in a dynastic context) in the phrase "An heir and a spare".
Spare as a noun (bowling):

The right of bowling again at a full set of pins, after having knocked all the pins down in less than three bowls. If all the pins are knocked down in one bowl it is a double spare; in two bowls, a single spare.
Spare as a noun (bowling):

The act of knocking down all remaining pins in second ball of a frame; this entitles the pins knocked down on the next ball to be added to the score for that frame.
Spare as a noun (Canada):

A block of school during which one does not have a class.

Spare as a verb (intransitive):

To show mercy. To desist; to stop; to refrain. To refrain from inflicting harm; to use mercy or forbearance. To preserve from danger or punishment; to forbear to punish, injure, or harm; to show mercy.
Spare as a verb (intransitive):

To keep. To be frugal; not to be profuse; to live frugally; to be parsimonious. To keep to oneself; to forbear to impart or give. To save or gain, as by frugality; to reserve, as from some occupation, use, or duty.

Examples:

"'Spare the rod and spoil the child."
Spare as a verb (transitive):

To deprive oneself of, as by being frugal; to do without; to dispense with; to give up; to part with.