There are various kinds of knife sharpeners available for maintaining the sharpness of knives.
A sharpening stone or oil stone is also known simply as a stone, and is a tool for maintaining the sharpness of knives. It consists of a carborundum stone, a block of ceramic, or a metal plate with embedded diamond dust. There are generally three 'grades' of stones: a coarse stone for rapid renewal of a badly damaged blade, a medium stone for general work and a fine or Arkansas stone to put the final edge to the blade. Many of these require the addition of an appropriate lubricant, such as oil or water, to carry away the "cuttings" from the edge while sharpening. The lubricant prevents the pores in the stone from becoming clogged up with the metal and grit created by the process.
For maintaining the edge on a knife, and not for intensive sharpening, the honing steel, also known simply as a steel, is the choice of professional chefs. These do not actually sharpen knives, unless they have fine diamond grains on them, but they are often mislabeled as "sharpening steels".
The steel was originally used to straighten the sharpened edge of a blade where use had pushed it to one side or another on a microscopic scale. This is preferred, as the metal is brought back to its sharper state quickly, often a matter of four or five strokes of the steel, by restoring the existing edge rather than stone-cutting a new one.
A steel usually consists of a hardened steel rod with many tiny grooves scored lengthwise. Seemingly paradoxically, they can also be made from a ceramic material specifically intended for sharpening hardened knives, as a standard steel can quickly become blunted. Another version is the diamond steel, with tiny specks of industrial diamond dust bonded to its surface. These are very effective for both sharpening and honing the edges of knives, but they tend to wear away at the knife far more quickly than do other kinds of steel.
To use a steel, the knife is drawn across roughly perpendicular to it, somewhat like a slow slashing motion, that both takes the knife across the steel while moving down the steel's length, alternating sides of the knife with each pass. Home cooking enthusiasts will find that an inexpensive (US$20) steel will bring many knives back to life in moments.
Holding the knife at the appropriate angle, about 15-25 degrees, to match the existing stone-cut angle, the first moves of the knife will feel kind of rough and resistant to the steel. The feel will diminish within five or ten strokes, even holding the same angle and pressure. That is the steel telling you the job is done. A good steel will extend the life between stone sharpening by many months in most cases.