According to an online medical dictionary* a menstrual cup is “A rubber cup that is inserted into the vagina and placed over the cervix to collect menstrual flow.”
This is what a menstrual cup looks like: 
It is a female hygiene product usually made of flexible medical silicone, thermoplastic elastome or rubber and is inserted into the vagina during menstruation to collect menstrual blood. It is an eco-friendly and healthier alternative that a tampon or sanitary pad, and on the contrary it also collects the blood rather than absorbing it. Blood only starts to have an odour when exposed to air and using a menstrual cup should be odour free. Tampons on the other hand absorbs blood with the help of a chemical absorbent and are more likely to cause bacterial infection, dryness of the vagina and bad smells.
If you are latex-sensitive take care to purchase a cup that is entirely made of medical silicone.
Menstrual cups are shaped like a bell with stem to assist with insertion and removal. A menstrual cup is held in place with suction and you can even swim while wearing a menstrual cup. It protects your clothing from stains during menstruation.
A cup can be used for up to 12 hours at a time, but normally you can empty it between 4 – 6 hour intervals and it is also recommended for ladies with heavier flow.
The majority is long-lasting and durable, but you also get disposable menstrual cups. A reusable cup can be used up to 5 years and even though the initial cost is much higher than tampons or other menstrual products the cost is beneficial over the long term saving you money and saving the environment with significantly less waste that would normally be associated with tampons or sanitary pads.
Menstrual cups are available in different sizes and there are a large number of new brands on the market to suite every individual preference.
* The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company.