As far as European cities go, Barcelona is a relatively easy city to navigate. Its natural mountains and seafront means that it has a gradual incline, and also is very compact. This does however mean that some of the city's sights are missed, as it's sometimes tiring walking uphill for half of the day, and downhill for the other half. The city's metro system is a great way to get around quickly, but you miss out on actually seeing the city, and often are disoriented when you get back up to street level (everyone does the same in rotating the map around a few times to double-check where you are!).
So in steps the tourist bus which solves all the previous problems we've mentioned. There are 2 companies in Barcelona operating this service (both following the same routes) but the one I recommend is the Barcelona Bus Turistic, which is the one run by the local council. This basically translates as there are more buses driving around the city, so you don't have to wait as long at the bus stops.
The Barcelona Tourist bus has 3 routes, but I would say only 2 are worthwhile - the red and the blue route (North and South routes). The 3rd green route only takes you to the forum building (conference centre which was a bit of a flop and cost the mayor his job in 2004). So if you concentrate on the red and blue routes, you can visit all of the city's sights, and have the added bonus of being over-ground as you do it, so you actually see the whole of Barcelona, too. Obviously during the spring and summer months it's great to be on the top with no roof and a nice breeze running through your hair while you take in the stunning architecture of Gaudi, the amazing Camp Nou football stadium, the magic dancing fountains at Placa Espaa, and many, many more attractions.
Many people always head for Plaa Catalunya - the city's main square - to start their bus route, but this is the most common mistake. As this is where the blue and the red cross over, and it's also where the Main tourist information office is, then there is the assumption that this is the starting place for the tourist bus - but the routes are just a loop, so it doesn't matter where you start or stop. A tip is to go down to the Port, for example, where there are always fewer people waiting around. You can buy the ticket on the bus, and also have the advantage that human nature means that as the people who are already on the bus see the waterfront and the port, they think it's a great place to get off (happens every time) and loads of people will pile off, leaving many free spaces.
David writes about Barcelona apartments and Cheap barcelona apartments