A trade mark can be merely used or adopted without seeking registration. However, prior to doing so, one needs to be careful that no other person has rights to the trade mark which you might be infringing. This includes not only identical trade marks but also those so similar which a member of the public might assume to be connected and so become confused. It is also a possibility that you could infringe someone's rights to a registered trade mark when used on completely different goods or services depending upon the similarity of the trade marks.
Once using a trade mark which is not registered, it is significantly more difficult to defend your rights against someone else using your trade mark or a similar one. This is called "passing off". In order to defend your rights, you need to prove that the trade mark is rightfully yours; that you have a reputation through using the trade mark; and that there has been monetary damage to you as a consequence of the other person using the same or similar trade mark. Passing off is often very expensive to prove and difficult to collect suitably strong evidence.