Shavuot, The Festival of Weeks....I am lifting most of this explanation from here with a few of my own thoughts...just a few though.
It is the second of the three major festivals with both historical and agricultural significance (the other two are Passover and Sukkot). Agriculturally, it commemorates the time when the first fruits were harvested and brought to the temple, this is why it is also known as the Festival of the First Fruits. Historically, it celebrates the giving of the Torah at Mount Sinai.
Shavuot is not tied to a particular calendar date, but to a counting of weeks from Passover, hence the name. Shavuot is also sometimes known as Pentecost, because it falls on the 50th day (pente...five...get it?). The Jewish people recognize that Passover freed them physically from bondage, but the giving of the Torah on Shavuot redeemed them spiritually.
Work is not permitted during Shavuot, it is celebrated like a normal Shabbat here in Jerusalem. It is also customary to stay up the entire first night of Shavuot and study Torah, then pray as early as possible in the morning. I can personally account for this, as the yeshiva outside our window was most certainly up all night...as were we. The book of Ruth is read, but there are varying reasons given for this custom. In my mind, however, it makes perfect sense given that she was harvesting the leftover grains in Boaz's fields.
It is common to eat dairy meals (remember, meals are either meat or dairy accordong to kosher restriction) during Shavuot. Some say it is a reminder of the promise regarding the land of Israel, a land flowing with "milk and honey." Others note that they had just received the kosher laws and dairy was the best option for their first kosher meals.
This boils down to one thing: expect people to be eating cheesecake today. Seriously.