It's observed across the world, and in some countries is a national holiday. Basically, it celebrates women that have made great achievements throughout history.
A bit of history:
The Socialist Women's Party within the United States (Yep, you read that correctly) declared National Women's Day a holiday in 1909. The original celebration date was February 28th. It was celebrated by women until 1913.
Now let's rewind back to 1910:
The Socialist International, which met in Copenhagen, decided that this whole Women's Day idea was rather a good one, and so they declared it to be an International holiday, though no specific date was set. It was created to assist women across the world in their fight for suffrage. All women attending the conference approved of this.
In 1911, Germany, Denmark, Austria and Switzerland declared March 19th to be marked as International women's day within their countries.
Then, on March 25th, a massive fire broke out in a New York City building where shirtwaists were made, taking out the 3 upper floors of the ten story building, and killing 141 people inside. Some leaped through windows, dying as they fell to their deaths, as others burned to death, or died from smoke inhalation.
Here's the really horrible part, though...
It wasn't your typical fire tragedy. This fire, known as the Triangle Fire (The Triangle Waist Company owned it), did not damage the walls or floors of the building - the building itself was fireproof. Indeed, the deaths of these people, at least 125 of which were young women between the ages of 16 and 23, were caused by fire spreading through the furniture and clothing scraps littering the area.
The media proclaimed the building a firetrap (only one fire escape in the building, and it was an internal escape, rather than external), and investigations into how this disaster were begun. Poor working conditions were once again brought into focus, and alluded to during future Women's Day Observances, both national and international.
Between 1913 and 1914, March 8 became the decided upon day to celebrate International Women's Day throughout Europe (Well, except Russia. They decided to be different and make their observance day the last Sunday of February.)
Then, in 1917, women within Russia when on strike for Bread(stability) and Peace on the last Sunday of February, which really ticked off the politicians. Four days later, though, their efforts were shown not to be in vain - the czar got kicked off of his throne, and women were given the right to vote.
Take that, Nicholas II!
And that's the history of it. What once was created to give women voting rights and better working conditions has grown into a day in which we celebrate all women that have made breakthroughs that help women achieve equal footing with men.
As an environmentally oriented site, then, who do I think should be celebrated?
A lot of people, actually. But I want you to learn about them on your own. They're that awesome.
Founder of the Green Belt movement, and winner of the 2004 Nobel Peace Prize. Through the Green Belt movement, over 45 million trees have been planted across Kenya, which has, in turn, created better nutrition and more jobs for women within the country. Labelled a crazy woman, she went head to head with government backed expansion projects and won. She never shied from a conflict, even though people told her to be a good woman and do what she's told.
Remember Chico Mendez, environmentalist and trade union leader that was assassinated because of his efforts to save the rainforests? She was his colleague, and later became the Minister of Environment for Brazil. She also created the Sustainability Party.
Stay at Home Mom turned Superhero.
Ok, I'm exaggerating, but as you can imagine, I find her personally inspiring. One day, she discovered that her son's elementary school was built on top of a toxic waste dump. The superhero part comes into play when she was able to get the federal government relocate 833 families that were in direct danger due to the 20,000 tons of chemical waste, by creating a community organization to fight this travesty with no prior activist experience.
So go do some web surfing, and learn about these magnificent women. I've given a brief summary of awesomeness, as well as starting links, but it's up to you to go in depth.