Following and unfollowing are important tasks on twitter. These tasks can be time consuming too. if you want to follow a huge number of users you cant manually go to each users account and click the follow button this is when bulk follow tools come in use.
People also want to unfollow people who are not following them in return , the list may be huge so it is better o use unfollow tools. Also if you don’t have time to follow all those following you you can use these tools.
Some popular effective tools are :

• friend or follow: I don’t like their interface too much and this task is itself very time consuming. But it displays a lot of other graphical info about your profile which is helpful
FriendOrFollow.com - Who's not following you back? Who aren't you following?
*Untweeps: This is a mass unfollow tool for unfollowing those users who are no longer active on twitter, so can have value to your follower following ratio.
Twitter Webapp Social Media Marketing - UnTweeps - Unfollow Twitter users who don't tweet often enough.
*Buzzom: By far the best tool I have ever used. IT works best for me . It is faster to flush or follow. This is really great an you should use this one
Social Media, Twitter, Facebook, Marketing, Branding : Buzzom Home