While most people are familiar with the standard black and white grid, crossword puzzles actually come in a number of permutations. The American-style grid usually has no more than one-sixth of its squares blacked out, and most of the letters will "check" each other. This means that getting most of the clues in one direction (across or down), will automatically supply the words for the other direction. British and Australian puzzles have a lattice-style grid, with the black squares placed so that only about half the letters are checked. There are also barred crossword puzzles, which have bold lines separating the squares, rather than black ones.
Crossword puzzles appeared in the United States in 1913. They became so popular that newspapers began using them, and word enthusiasts began creating them in huge numbers. They jumped the pond in 1924, with their first appearance in the United Kingdom. Nowadays, they are available in most newspapers, in some magazines and in puzzle books all over the world.
The most familiar variation on the puzzle is the standard "quick" clue. These are simple clues with one-word answers. Acronym clues are also used, as well as abbreviation clues and indirect clues.
The cryptic crossword, in which the clues themselves are little puzzles, are most common in the UK. Some crossword puzzles are even written so that every answer is in a foreign language. There are also double-clue crosswords. These are puzzles designed so that the puzzle worker can either go by a list of simple clues, or turn the page and use the cryptic clues, thereby increasing the puzzle's difficulty, but enabling either beginners or experts to work the same puzzle.
Cipher crossword puzzles have numbers in the clues, and the puzzler must break the cipher code to read the clues and solve the puzzle. One diabolical twist on them is the UK variation called a diagramless puzzle. These puzzles involve getting all the clues answered, and then placing them on the puzzle grid so that the answers check each other properly.
A new puzzle similar to the crossword is the wildly popular Sudoku. This involves placing the digits 1 through 9 in a grid in such a way that every row, and every 3-by-3 square, contains each of the numbers, with no repetition. As long as humans like to play with words, crossword puzzles will, no doubt, have a place in the puzzler's favorites list.