Ramadan last for 30 days and is observed by most people and all places of business. For the tourist, there are pros and cons.
During Ramadan people fast (not even water) from sunrise to sunset then have a feast (pigging out). Sunset occured around 5:00 while I was there. This means schools are let are let out early, shops close around 3:30-4:00 (depending on how long it takes them to get to their home) and museums also close at this time. But, thankfully not all restaurants. As a side note places that are usually open very late will reopen around 6:30-7:00.
This means that if you have guide, you may be rushed because he wants to get home before 5:00. It also means heavy traffic (like it isn't always) and fewer hours at the sights.
On the other hand, I found this also meant less crowds in the Egyptian Museum as well as other tourist sites. I didn't know why, but I appreciated it.
Like our Easter, Ramadan does not occur on the same date, so if it matters to you, inquire before you make your plans.