Probably needs little introduction. Rush hour is the time when travelling in London is absolute hell. It varies by location but the morning rush can start as early as 7am (noted at Stratford Station) and end as late as 10am; and the evening rush seems to go from about 5pm to 6:30pm. (Please note any conflicting experiences!) Of course, weekends are different and don't have such a well-defined rush hour.
It can be more pleasant if you have a little insider knowledge though.
For example, some bus routes/directions get horribly clogged yet others are manageable (the 220 southbound from and to Hammersmith Station is much more usable than the northbound; I wouldn't even consider using the latter during rush hour. I've overtaken three or four buses and stayed overtaken, on foot, on a 20-minute northbound walk up Fulham Palace Road.)
Another useful thing to know is Where To Get On to a train at a given tube station for the best chance of getting a seat.
Also note that however many Tube trains you have to let pass before you can get on, at least with the Tube you know that your train isn't going to get stuck in traffic, unlike buses, and the only arbiters of whether there's room in the train are the passengers who're already on there, and they're less likely to stop you squeezing on than is the driver of a bus, or conductor of a Routemaster.
Finally, Tube Etiquette is even more important at rush hour than other times of the day. Think of the poor people who choose to travel during rush hour twice a day, five days a week; a bit of politeness can do wonders for their day.