For advertising creatives, designers and art workers, portfolios are a necessity. If you don't have one, start putting one together as soon as possible. In other roles you’re not expected to have a portfolio but that doesn't mean you can't have one.
Your portfolio, or your "book", is a continually evolving collection of the best work that you have done through school, college, university, at agencies and on personal projects. Your aim is to get it in front of as many professionals and employers as possible so they can see your talent, give their feedback and offer you an opportunity.
Ultimately, your portfolio will play a big role in getting you hired for placements and graduate roles. So it's really important to show off how awesome you are and show the full range of your skillset. Your portfolio needs to grab peoples' attention and leave a lasting impression.
You should consistently update a physical version of your portfolio. Keep it clear and concise. Quality is important; three great ideas and executions and far better than eight mediorcre ones. Don't try to cover all bases, focus on what you are really good at and show how this works in different ways, on different media and solving different problems.
Your work should be online. There’s really no question about it. For one, employers are likely to want a taste of your work before agreeing to give you an interview or a book crit. But just as importantly, you never know when you’re going to need it. You might run into a creative director at an event or get trapped in a lift with your favourite designer. Unless you want to carry around your physical portfolio with you at all times, online is essential.