So what's an ordo? An ordo is a calendar that provides direction for the liturgies for the day. It's primarily intended for clergy and religious, although lay faithful who assist with liturgical celebrations or pray the Liturgy of the Hours use it, too. You can expect to find one in almost every sacristy- it tells the celebrant the particulars of that day's Mass such as which one to say (or which Masses he is permitted to say), the color of the vestments, the rank of the day, and which readings may be used. From personal experience, it becomes especially handy if the ribbons happen to fall out of the Roman Missal or the Lectionary right before Mass starts!
Homilists and others who have to plan ahead for liturgical events use the ordo to help with their duties. These are people like the music director or those who read from the Lectionary. It's especially helpful to the sacristan, so (s)he knows which color the altar cloth (and other decor) should be, and which vestments should be prepared for the celebrant.
To give you an idea of how this works, here's a scan of the page for November 7-10 in 2012. Clicking on it will make it easier to read:
If you look at November 8, it is Thursday of the 31st week in Ordinary Time. The ordo says that any set of prayers can be used for Mass that day, however the prayers for the 10th or 12th Sunday in Ordinary Time are suggested. The liturgical color is green, and the readings for the Mass are from #488 in the Lectionary and are listed there. The V3R3 abbreviations mean that a votive or ritual Mass (such as a funeral or another Mass for a specific need or occasion) may be celebrated on that day at the celebrant's discretion.
The next day is a little different. November 9 is the Dedication of the Lateran Basilica, which is celebrated as a feast. The Mass prayers must be taken from the Common of Dedication Mass. The liturgical color is white, and the Gloria must be recited or sung. The readings for the Mass are from #671 in the Lectionary. V1R1 indicate that a funeral may be celebrated on that day, but all other types of ritual or votive Masses may be said only with permission from the diocesan bishop.
So if you attended the daily Mass on that Thursday, it would have been a shorter daily Mass with only one reading before the Gospel, no Gloria or Creed, and less music (if any). Because of the feast day, the following day's Mass would have been more like a Sunday Mass with two readings before the Gospel, the Gloria would have been said/sung, and generally more music (depending on the resources of the parish or community).
Something else you should know is that each diocese has it's own liturgical calendar. It's based on the General Roman Calendar, but with a few adaptations including local celebrations such as titular feasts, diocesan patrons, and anniversaries of dedications that are important to the area. Many religious orders also have their own general calendars which will vary slightly to incorporate saints and other celebrations important to them. (Often specific provinces or congregations within larger religious orders will have their own regional adaptations, too.) For example, if you catch the daily Mass on EWTN and on the same day happen to attend Mass at your parish, every once in a while you'll notice a difference in the day's saint- that's because the Franciscan Missionaries of the Eternal Word follow a Franciscan ordo, while your parish probably follows the diocesan ordo. I noticed a number of similar differences when I studied at Saint Meinrad. The particular Benedictine ordo that we followed has the Feast of St Benedict on March 21 (the day he died), however the rest of the Church celebrates his feast day on July 11. So for several years, I celebrated St Benedict's feast day twice, once during the spring semester at school, then again at home during the summer.
For many years, Paulist Press has been the most popular publisher of ordos in the United States. All the dioceses are grouped into regions, then a specific book is published annually for each. The regional book also contains a necrology of the clergy in each diocese, so that each can be remembered on the anniversary of the day he died.
As technology continues to advance, so do the available formats of the ordo. Some ordos are now available online, such as this one for the Jesuit provinces in the United States. Recently the deacon at my parish told me about the new "Ordo App" on his iPhone from Paulist Press. Personally, I'm sticking with the classic book version for now!