Selenium multiple tabs at once

If there is a link that opens up a new window/tab, then you can use driver.switchTo().window();

However, if you want to run something on multiple windows, then I recommend having multiple instances of webdriver. It is much easier to manage, and is supported (There are workarounds on opening a new tab/window, such as pressing a hotkey that opens a new window, but they aren't supported).

If you are wanting to have multiple threads all act on the same driver instance, but different tabs, that is NOT possible.

To open multiple tabs:

driver = new ChromeDriver();
IJavaScriptExecutor jscript = driver as IJavaScriptExecutor;
for (int i = 0; i < 10; i++)
{                
  driver.Navigate().GoToUrl(this.baseURL);      
  jscript.ExecuteScript("window.open('{0}', '_blank');", this.baseURL);
}

Swich between them:

for (int i = 0; i < driver.WindowHandles.Count; i++)
{ 
  driver.SwitchTo().Window(driver.WindowHandles[i])]);
}

It is possible to switch between individual tabs without having multiple browser instances.
There is a difference how web driver handles different windows and how it handles different tabs.

Case 1:
In case there are multiple windows, then the following code can help:

//Get the current window handle
String windowHandle = driver.getWindowHandle();

//Get the list of window handles
ArrayList tabs = new ArrayList (driver.getWindowHandles());
System.out.println(tabs.size());
//Use the list of window handles to switch between windows
driver.switchTo().window(tabs.get(0));

//Switch back to original window
driver.switchTo().window(mainWindowHandle);

Case 2:
In case there are multiple tabs in the same window, then there is only one window handle. Hence switching between window handles keeps the control in the same tab.
In this case using Ctrl + \t (Ctrl + Tab) to switch between tabs is more useful.

//Open a new tab using Ctrl + t
driver.findElement(By.cssSelector("body")).sendKeys(Keys.CONTROL +"t");
//Switch between tabs using Ctrl + \t
driver.findElement(By.cssSelector("body")).sendKeys(Keys.CONTROL +"\t");

Detailed sample code can be found here:
http://design-interviews.blogspot.com/2014/11/switching-between-tabs-in-same-browser-window.html