A helmet protects the head from abrasion, sharp objects, and relatively light impacts or glancing blows. No helmet will save your life if your head receives a really hard blow. The shock wave of a severe impact is transmitted through the brain, snapping nerves and blood vessels. Alternatively the neck can be broken or crushed.
There are two main types which will be mentioned here; open-face and full-face. (The third type - the "half-shell", is sometimes seen being worn by riders of vintage motorcycles. This type offers the least protection).
Some people don't like full-face helmets saying they feel claustrophobic. If this is you, for heaven's sake either get a clip-on visor or some goggles. Why? Because when an insect smacks you between the eyes at motorway speeds, it hurts! Don't ask me how I know.
One of the disadvantages of a full-face helmet, even with the latest ventilation designs, is that of the visor fogging up in rainy conditions. I tried "Fog City" visor inserts in my HJC helmet with some success; my Shoei helmet came supplied with a "Pinlock" insert which does much the same thing.
An irritating quirk I found with my Shoei was that when it rained, cold water would somehow find its way down my neck - most unpleasant! I solved this by buying a "wind-stop" - an elasticated skirt which I attached to the bottom of my helmet using double-sided tape. You could also wrap a strip of towelling around your neck to keep rain and cold air from getting past.
(Oh and by the way, if you have to carry an additional helmet for a pillion passenger, do not ride with it attached to the helmet holder that most bikes have. This holder is designed to be used when the bike is stationary. And don't ride with a helmet slung over your arm either - it can get caught between the handlebars and the tank. The best way to carry an extra helmet is either in a pannier, top box, or tied to the pillion seat with strong cord or elastic straps.)
All helmets must meet relevant safety standards. In the US, this is the DOT standard set by the US Department of Transportation. Any helmet meeting this standard must carry a DOT sticker.
In Europe there are commonly two different sets of approval initials: BS and ECE.
BS6658-85 is the BSI standard. Any helmet carrying this label is legal for road use on a motorcycle. The standard is divided into three categories: Type A/FR (a red label) for helmets with a fire-resistant lining, Type A (a blue label) for high-performance helmets and Type B (a green label) for lower-rated helmets.
ECE is the Economic Community of Europe approval system.
ECE 22-05 is legal for road use on a motorcycle in the UK, but the ECE 22-04 standard is older and less stringent, so is not.
Snell also has a set of standards for motorcycle helmets, the latest being Snell M2010. (William Pete Snell was a motor-racing driver who was killed in a crash in 1956 when the helmet he was wearing failed to adequately protect his head. The non-profit SNELL organisation was then set up to improve the effectiveness of helmets.)
Helmets do not have to meet the SNELL standard to be legal, but if it is both DOT and SNELL approved, it will offer the highest standards of protection. However more protection usually means more weight, and more cost.
How to choose a helmet
Firstly, buy a new one. Buying a used helmet is a waste of money because you can't tell if it has been dropped or not and damaged on the inside.
Having decided on whether you want a full-face or open-face helmet, there are several brands for you to choose from. I've personally tried Nolan, AGV, Schuberth, HJC and Shoei. Of all of these I've found the Shoei to be the most comfortable for my particular head shape. The best advice I can offer here is to try several brands to see which one fits you the best.
It is important to choose a correctly-fitting helmet. If it is too big, the wind can get underneath it and pull it upwards, causing neck strain. If it is too tight, it can cause headaches. It should feel slightly tight when you first try it on - bear in mind it will loosen slightly with use. A good indication of fit is to put the helmet on, and get the salesperson or a friend to hold it with both hands while you try and move your head around inside. You should not be able to. Also ensure that the lower portion of a full-face helmet fully covers your jaw, and does not obstruct your downward vision too much.
Some helmets are also heavier than others; the lighter the helmet, the less it will pull on the neck in high wind speeds. Helmet weight has decreased significantly since the introduction of carbon compounds, especially when compared to the older fibreglass models.
Follow the manufacturer's instructions; each manufacturer has different opinions on how a helmet should be cared for, so it is always worth checking their website.
When cleaning your helmet, use mild soap and water. Avoid any petroleum-based cleaning fluids as these could damage the graphics. Strong cleaning agents could also cause the helmet component materials to decompose.
Do not drop your helmet. Not only could you damage the protective coating, but the foam padding inside could become distorted. You won't see it, but the damage is there. And wearing a damaged helmet is only marginally better than no helmet at all.
Don't keep your helmet near fuel, cleaning fluids, exhaust fumes, or excessive heat. Helmet materials can react chemically to these and the damage is most often invisible.
Do read the manufacturer's instructions about painting, decorating, or applying decals to your helmet. Some helmet compositions can be damaged if painted or if decals are applied.
Never hang your helmet on the motorcycle's mirrors or turn signals, or place it on the seat. It can fall off (see "do not drop" above).
Always place your helmet on a flat surface. i.e. on the ground, shelf, or securely on a rack.
One final point; a helmet is of no use whatsoever if it is worn without the strap being properly fastened.