Menstrual cups are a green alternative to tampons and pads (sanitary napkins). They are worn internally similar to tampons. (A green alternative to pads would be cloth pads which can be washed and re-used.)
- Save money - stop purchasing tampons and pads every month. A menstrual cup is an investment that will pay for itself in about 4-5 months, and after that you are saving money every month!
- Reduce waste - menstrual cups are reusable and can last for up to 10 years. No more landfill space for sanitary products!
- Greater comfort - because cups are not absorbent, they are not drying like tampons. Cups have not been associated with TSS, either (unlike tampons). Cups have no strings attached!
- Perfect for travelling - save packing space and never get caught short. Also great for countries where tampons are hard to find, e.g. China
Because using a menstrual cup means you have to carefully remove a small cup of blood from your own body, menstrual cups are not really for the squeamish. But if you think about it, being a woman is not really for the squeamish, either. :) Some women report that when using a menstrual cup, for the first time in their lives they actually enjoy their period.
If you are particularly keen, you can use your menstrual blood as fertiliser for a garden.
 How does it work?
A menstrual cup is a small silicone or rubber (latex) cup. You fold it and insert it into your vagina. It will unfold, creating a suction seal with the walls of the vagina, and then happily sit there by itself collecting blood. If it is inserted correctly you won't feel it at all.
When it is full, or when you want to, you can remove it to empty it and replace it again. Blood can be tipped down the toilet or bath/shower and the cup can be cleaned with just regular soap and water (or wiped with toilet paper if running water is not available). No, it's not sterile -- but neither are tampons or pads. You may also boil silicone cups or use chemical sterilizing solutions.
If you are a new menstrual cup user it is advisable to start using it when you know you will be at home, for example. It can take some practice to get used to removing and inserting the cup (especially when it is slippery and full of blood). Doing it in the shower can be a good idea because any mess is washed away immediately. How often the cup needs to be emptied depends on how heavy your flow is, but twice a day (every 12 hours at the manufactures' recommendation), can be often enough.
 Do women really do this?
Yes! And it is awesome. Even if you have a circle of really green-minded friends, you still might not know anyone who uses a menstrual cup, as it's also not necessarily something that is often discussed.
There are some wonderful online communities of menstrual cup users, e.g. menstrual_cups on livejournal (also Flickr). (See also the memorable posts.) These are women who have all tried using menstrual cups and have experienced any problem or embarrassment you can imagine. They are a great resource to share both your trials and successes with.
If you've never heard of a menstrual cup before then it might seem shocking or crazy to you at first. But just keep the idea rolling around in your head; think about it the next time you are bleeding and unwrapping another tampon, or in the supermarket scanning the shelves and shelves of commercial products. Have a look at one of the communities and read a few posts. Let the idea become normal to you.
Yes, you can use a cup if you have given vaginal birth. Yes, you can use a cup if you are a virgin or teenager and have never given birth. Yes, you can use a cup if you have had a Caesarian section. Most manufacturers make two different sized cups to accommodate the differences between women, but the criteria vary so read the descriptions carefully and/or chat with online communities before buying. Some women buy more than one brand of cup to find the best "fit" others are happy with the first one they try. Some women just buy them in different colours for fun! Believe it or not, having your period can be fun!
 Where can you buy them?
Many people buy cups online. There are various manufacturers: The silicone DivaCup, made in Canada, is a long cup. The silicone Femmecup, made in the UK, has one size and is the lowest cost. The latex Keeper and silicone Moon Cup, made in America, is the only latex cup. The silicone LadyCup and Colour Cups, made in the Czech Republic, is available in 7 colours. The silicone Lunette, made in Finland, has a flat stem. The silicone Mooncup, made in the UK, has a thick rim.
Please note that the rubber/latex cups are not recommended for people with sensitive skin/membranes or allergies. It is possible to develop a latex allergy with time and exposure even if you did not have one previously, so if you have been using a Keeper for a few years and recently started to get itchy, it could still be a cup allergy. Switching to silicone usually fixes the problem, as medical-grade silicone is hypo-allergenic.
Once you've decided about rubber vs silicone, the next best thing is probably to look for one that will fit your body.