The term "drip irrigation" describes the application of water not only by drip emitters but also by microsprays. Both of these have two traits in common: they operate at low pressure, and they deliver a low volume of water compared to standard sprinklers. Because the water is applied slowly on or near the ground, there should be no waste from runoff and little or no loss to evaporation. You position the emitters to deliver water just where the plants need it; you control penetration by varying the time the system runs and/or the emitters' delivery capacity (rated in gallons per hour--gph). You can also regulate the volume of water delivered to each plant by varying the type and number of emitters you set up for each.
Besides water conservation, the chief advantage of drip systems is flexibility. You can tailor them to water individual plants by providing each with its own emitter(s); or you can distribute water over larger areas with microsprays. A standard layout might include hookups to two or more valves and many kinds of parts. Because the lines are above ground (they're easily concealed with mulch) and are made of limber plastic, changing the system is simple: just add or subtract lines and emitters as needed.
Your drip system can be simply attached to a hose end or screwed into a hose bibb. Or, if you prefer, you can connect it permanently to your main water source.
EMITTERS FOR DRIP SYSTEMS
Emitters vary in shape, size, and internal mechanism, but all operate on the principle of dispensing water slowly; flow rates range from 1/2 to 2 gph. You insert the emitters directly into 1/2- or 3/8-inch drip irrigation tubing or into thinner microtubing positioned to run from the larger tubing to each plant. Non-pressure-compensating emitters (the standard kind) work well on flat or relatively level ground and with lines less than 200 feet long. But when water pressure will be lowered by gravity or friction (on hillsides or with long lines), opt for pressure-compensating emitters. These deliver the same amount of water throughout the system.
Emitters also come factory-installed in polyethylene tubing; these are often referred to as "in-line" emitters or emitter lines. Spaced 12, 18, 24, or 36 inches apart, they deliver 1/2, 1, or 2 gph and are available in non-pressure-compensating and pressure-compensating versions. Some emitter lines are infused with a small amount of herbicide to prevent root intrusion; these can be buried and used to water lawns.
The standard emitters simply drip, but sorts that deliver water in other ways are available as well. Misters produce a fine spray--a good way to increase humidity for plants like fuchsia and tuberous begonia. Microsprays are low-volume equivalents of standard sprinklers.