The nine days are the days between the beginning of the month of Av, and the date when the temple in Jerusalem was destroyed.
I read an interesting piece in the shul bulletin this past Shabbat. It said that usually, Jewish mourning rituals go from more intense to less, the funeral, the seven days of shiva (stay in the house, no laundry, etc), then the 30 days of shloshim (no live music, no parties, no shaving), and the eleven months of saying kaddish (the Jewish prayer for the deceased). However, in the case of mourning for the temple in Jerusalem our mourning gets more and more intense. We start with the three weeks, then the nine days (where eating meat is forbidden, as is listening to live music, laundry, bathing for pleasure), and then the 9th of Av (which is a fast day).
I observe the nine days pretty stringently. I don't eat meat, I don't do laundry, listen to live music (or any music for that matter), or swim (that sucked when I was at camp). One of the other observances is to "refrain from purchasing anything that lifts your spirits." Ben-Yehudah of Esser Agaroth suggests that this means that we shouldn't buy the Harry Potter book until after the 9 days. Which sucks.
JoeSettler posts on the Muqata blog suggesting that a good 9 days activity is to attend the Cohanim and Leviim conference in Israel. Sadly, I am neither. A Mother in Israel has a "kosher cooking carnival" of foods that are permissable (read: not meat) during the nine days. Reb Barry Leff suggests that if you live in Israel, now is a good time to go out to your favorite meat restaurants, as they will be mostly empty. While I get what he's saying, I don't really know of anyone who keeps strictly kosher but eats meat during the nine days. Also, don't most meat restaurants change their menus and charge absurd prices? (Ahem, Estihana)
At any rate, there are a lot of blog posts about biblical stuff relating to the 1st of Av (like when Ezra arrived in Jerusalem from the Babylonian exile) or the torah readings for this week (courtesy of DownHome Davar Torah), but I woke up feeling like I got hit by a train, so I'll let you find them yourself.