DRY is a Principle used in Software Development, used to reduce the repetition of all kinds while coding. Frameworks like ROR, Grails, Spring, etc,. uses this principle.

1. DON'T REPEAT YOURSELF (DRY)
DRY is a kind of principle used in software development, (software
engineering) to reduce repetation of information, and it is mostly used in
multi-tier architecture. It states that “the knowledge we are putting should be
unique, only once, unambiguous and authoritative”. It clearly says that
duplication in logic should be eliminated via abstraction and duplication in
process is eliminated via automation, hence DRY is sometimes called as
“Duplication Is Evil”. This principle had been formulated by Andy Hunt and
Dave Thomas in their book called “The Pragmatic Programmer”. They apply it
to include 'database schemas, test plans, the build system, and even
documentation'. When DRY is applied successfully, a modification of any
single element of a system does not require a change in other logically
unrelated elements. Additionally, elements that are logically related all
change predictably and uniformly, and thus kept in sync. They alternatively
used code generators instead of methods and subroutines in the code.
DRY vs WET
Violations of DRY and referred to as WET solutions, WET commonly
taken to stand for either “Write Everything Twice”, “Waste Everyone's Time”,
“We Enjoy Typing”.