A test tube is a small vial used in lab work. While there is some range in the exact design of test tubes, they are generally in the shape of a narrow tube with an open top and a rounded bottom. Of course, you know this, as test tubes are a part of our cultural literacy, and have become an icon of the scientist and the inventor. If you do an image search for 'scientist', the majority of them will be holding either a test tube or a beaker, the test tube's big brother.
Test tubes are traditionally made out of glass, as glass resists both heat and corrosion, important when you are heating volatile chemicals over an open flame. Modern test tubes are generally made out of borosilicate glass, such as Pyrex. The rounded bottom and straight sides of the test tube helps reduce residuals from the samples collecting in the tube, and the height of the tube's 'stem' prevents spillage gives a surface for clamps and racks to attach to.
Test tubes are generally identified by volume first and dimensions second. Test tubes come in a wide range of sizes, but it is common to see them ranging from 10 to 40 milliliters in capacity, 13-20 mm in diameter, and 100 to 150 mm in height.
Test tubes may be differentiated from culture tubes, which are used for living biological samples. While these look like test tubes, they are often plastic as they do not need to hold up to heat and chemicals. Culture tubes are therefor often disposable and much cheaper than traditional test tubes.
A large test tube (30 ml range) may be called a boiling tube. The extra size allows for rapid boiling and the accompanying expansion of gases, which a traditional test tube may be unable to handle without popping a cork. If you see a mad scientist holding up a test tube, it is most likely in the boiling tube size range, as small test tubes tend to disappear in the average person's hand. Boiling tubes are a much better scale for dramatic representations of science gone amok.
In the past 40 years 'test-tube' has become an adjective meaning 'made in the lab', as in the phrases 'test-tube baby' and 'test-tube meat'. This generally has a pejorative connotation. Note that the adjective should by hyphenated!
A complete test tube set up should include not only test tubes, but test tube clamps, test tube racks, and probably a Bunsen burner.