In Sudoku puzzle there will be cells that have only one possible candidate number, this is called a naked single.
A Naked single means we can safety eliminate that number from all other cells in the column, row and square, if there is only one cell that a number 9 for example can go in it is a Naked Single.
When there is only one candidate for a row, box or column but it is hidden amongst other candidates. In this example you will number 6 is the Hidden Single
If an pair of candidates are contained in 2 cells in a row, column or square then they are the only numbers that can fit in those cells. For example . Here you will find 1 and 8 appear together with no other candidates to for a Naked Pair.
When two cells in a square, row or column contain a pair of candidates that are not found in any other cells but are hidden amongst other candidates you can exclude the other candidates. In this example you will fund that 6 and 9 only appear together in two cells but are hidden amongst other candidates.
Naked Triples & Naked Quads:
The same principle that applies to Naked Pairs applies to Naked Triples & Naked Quads
A Naked Triple occurs when three cells in a group contain the same three candidates and no others. The cells in a Naked Triple don't have to contain every candidate of the triple. If these candidates are found in other cells in the group they can be excluded.
A Naked Quad occurs when four cells in a group contain no candidates other that the same four candidates.
In each Sudoku a value can only exist once in each column, square and row, an X-Wing is a pattern formed where there are two lines/columns, each having the same two positions for a candidate number. When this occurs then the number must be assigned to one of these two cells. We have filtered out other candidates on this example and just left the number 6.
Sword Fish is a variation of X-Wing but has 3 rows and columns instead of the 2 in X-Wing. So a swordfish is found when for three rows, there are two or three possible squares in which a particular number can be placed, and for all three rows these squares lie in the same three columns. In this case, this particular number can be eliminated as a possibility for all other squares in those three columns.