What are Governor Limits in the Salesforce?

As we know, Apex runs in multi-tenant environment, i.e., a single resource is shared by all the customers and organizations. So, it is necessary to make sure that no one monopolizes the resources and hence Salesforce.com has created the set of limits which governs and limits the code execution. Whenever any of the governor limits are crossed, it will throw error and will halt the execution of program.
From a Developer's perspective, it is important to ensure that our code should be scalable and should not hit the limits.
All these limits are applied on per transaction basis. A single trigger execution is one transaction.