Trademark is an instrument for the protection of intellectual property. It can protect for example the logo (text or graphics), trade name, company name, product name or service. Typically it protects a brand - so anything that clearly identifies the company and its products and differentiates them from competitors.
Registered trademark confirms exclusive rights to your trademark - denoted by R, TM or SM, and significantly increases the burden and protective force for violations of the rights owner. Registrations are valid for typically 10-15 years (depend on the country), can be renewed repeatedly. You can register a trademark on the territory of a particular country (country), the community countries (eg. EU) or internationally.
If you own a trademark, no one is allowed to use identical or confusingly similar designations without your permission. You have the right to grant someone a license to use your trademark.