Barcelona´s public transport system is good. Trains, trams, buses and Metro are all linked, which means you can use multiple-journey tickets like the T10 on all forms of transport. The system tends to be safe, clean, frequent and reliable so you will have no problems getting around the city. Be aware, though, that it becomes crowded during peak times and that the Spanish have little awareness of the people around them. Expect your personal space to be much smaller during rush hours. Bear in mind that it is proper etiquette to let older people and pregnant women on before you – doing so will save you the embarrassment of being shouted at by a little old lady (this does happen from time to time).
The city center is actually not that large, and although a majority of people use the public transport system to get to and from work and school, walking is a completely viable option for most. For touristic purposes it is quite easy to walk about without becoming too tired, and we highly recommend using the tourist bus, too.
Public transport at night is more intimidating than during the day. There is very little violent crime in Barcelona, but there are still enough pickpockets around to make it worth being watchful. Small precautions like only having enough cash on you to last the day, keeping your hand on the zipper of your bag, and looking alert will usually be enough to keep your possessions safe. In peak times, be aware for any unusual jostling – it could be a sign that your bag is being unzipped bit by bit.
Catalans are more reserved than other Spaniards, but overt staring and commenting (usually from men to women) takes place occasionally. If you are a woman travelling on public transport alone at night, you might be approached. Usually just giving a blank look or simply ignoring the offender is enough to curtail the harassment.