Showers are commonplace in Australia too (so you can bash us along with the Americans if you like
).
Usually a close friend or Sister hosts the party. Its to celebrate a wonderful impending event, not 'soley to get gifts' as some have said. In what way is it different from having a birthday party?
Often guests will be asked to bring some cake or another item to contribute to afternoon tea. Its usually just women (and sometimes their daughters), although my brother and SIL had a BBQ to which all their (male and female) friends were invited.
Usually the hostess will have a list of gift ideas for people who ask (things like bibs, wipes, cot sheets etc etc), and there is not expectation that everyone will bring a gift (although who wouldn't want to! Its exciting!)
If I go to the shower of a friend and give a nice gift, I don't then feel obliged to give another gift once baby is born! If Its a close friend or relative I might, but its certainly not expected!
Bridal showers are also fairly common - I had two in fact! Both thrown for me - one by a friend of mine, and people brought little kitch items like woden spoons, graters etc, and one thrown by my Mums best friend for all mu mums friends, because they wanted to celebrate and most weren;t invited to the wedding. That on was a 'Pantry tea' and all the women brought me their favourite recipe and ingredients to make it. It was really epecial, and I don't think tacky at all!