When purchasing a bike lock, take your time and think how secure the area where I will leave my pride and joy is. If you are locking your bike in a different place each day, it may be best to use a high security bike lock as they use hardened materials, but this means they're heavier, bulkier and a bit more expensive. If you are locking your bike up at a place of work maybe a chain lock will do. But always make note: That stronger the Bicycle Lock safer the bike.
It's important to remember that no lock is completely thief proof. They come in a range of styles with different levels of security. With bicycle locks you generally get what you pay for, so it's well worth spending a little extra. If you are leaving your bicycle for long periods a Heavy Duty D-Lock is the best option to consider. You may need to compromise so a high security chain lock may be the best option, which are more flexible and a little lighter. Take note of the security standards that are available, Gold, Silver and Bronze standards, this gives a good guide of the strength of the bike lock.