What does it mean to be bilingual?

Bilingualism is the ability to use two languages with equal fluency, and to sound like a native in both. Young children are naturally designed to acquire what ever language(s) they are regularly exposed to. Although adults can study a second language to a high, even fluent, standard, they rarely manage to avoid a foreign accent. That's why true bilingualism has to start early in life - and why you don't need to be 'good at languages' to be bilingual.

The language you speak is closely bound up with your sense of identity, and how you view the world: being bilingual can make you feel at home in a wider set of social situations, and can give you two slightly different ways of looking at things.
Even where two languages are quite similar and you can function perfectly in either of them, things feel different in different languages. Robin puts it like this: "I feel a slightly different person speaking (or thinking) German than English - like everything's slightly more focussed."
It's rare to come across people who are not glad to be bilingual. Letizia in West London says that her children "are very proud to be half English and half Mexican and to be able to speak two languages."
Web reader Sandra sees practical benefits too: "I'm quite proud of being able to speak and understand Polish, as I know it will help in the future - now that Poland are part of the EU, maybe more people will learn it."
Speaking two languages is thought to increase cognitive abilities. In other words, bilingual children often get better mark! Bilinguals are more employable, and earn more on average than monolinguals. They're even healthier in old age! A study at the University of York in Canada in 2004 suggested that speaking two languages can help keep you mentally agile. Bilingual volunteers had faster reaction times than their monolingual counterparts and were less likely to suffer from mental decline in old age.
Bethan from Llanrug believes that bilingualism for its own sake is positive: "Children who are raised in a bilingual household are proven to do better at school as well as being more tolerant of diversity and minorities. In today's climate this can only be a good thing."
New parents who are considering bringing their offspring up to be bilingual will find plenty of information and advice on the internet. The excellent Nethelp site contains a wealth of invaluable personal experience and handbag.com has a guide to the different approaches.
If you're only fluent in one language and are feeling jealous, don't despair. You don't have to be fully bilingual to feel the benefits of a second language. Harpal Singh from Glasgow was inspired to learn Gaelic by the late Radio Scotland presenter, Ali Abbasi: "Learning Gaelic makes me feel more Scottish and I recommend that everybody at least tries to pick up a few words. Tapadh leibh!"

For further reading:
Growing up with two languages by Una Cunningham-Andersson and Staffan Andersson.