Hi Shekhar, Selenium commands are basically classified in three categories:
Actions: Actions are the selenium commands that generally manipulate the state of the application. Execution of Actions generate events like click this link, select that option, type this box, etc. If an Action fails, or has a bug, the execution of current test is stopped. Some of the most commonly used Actions commands include: open (url), type (locator,value), click (locator), focus (locator), highlight (locator) etc.
Accessors: Accessors are the selenium commands that examine the state of the application and store the results in variables. They are also used to automatically generate Assertions. Some of the most commonly used Accessors commands include: storeTitle (variableName), storeValue (variableName), storeLocation (variableName), storeTable (variableName), storeAllButtons (variableName) etc.
Assertions: Assertions are the commands that enable testers to verify the state of the application. Assertions are generally used in three modes assert, verify and waitfor. Some of the most commonly used Assertions commands are: verifySelected(selectLocator, optionLocator), verifyAlert(pattern), verifyAllLinks(pattern), waitForErrorOnNext(message), waitForAlert(pattern) etc.