Many people have questions about a medical condition called phlebitis. Phlebitis means inflammation of a vein. Most of the time this is a "sterile" inflammation, which occurs in the absence of an actual infection (an exception might be after having an intravenous catheter that becomes infected). The inflammation of the vein may be associated with a blood clot, a condition known as thrombophlebitis. The superficial veins, such as varicose veins of the leg or arm may be involved or even the deeper veins found near the muscles. A blood clot of a deep vein, a DVT, can have life threatening consequences.
Most episodes of superficial phlebitis respond to elevation of the extremity, warm soaks, anti-inflammatory medication such as non-steroidal anti-inflammatory dugs such as ibuprofen or aspirin. Sometimes, a superficial clot can spread to a deep vein so it is very important to see a physician if you have a painful vein.
For an excellent review, see this article on WebMD: http://www.webmd.com/a-to-z-guides/phlebitis