This is one of the most frequently asked questions from LoadRunner newbies. If we go by the literal meaning from Dictionary.Com both are same but from LoadRunner’s perspective there is a slight difference.
All the vusers in a particular scenario are called Concurrent vusers. They may or may not perform the same tasks. On the other hand simultaneous vusers is more to do with rendezvous points. When we set rendezvous points we instruct the system to wait till a certain no of vusers arrive so that they all can do a particular task simultaneously. These vusers performing the same task at the same time are called Simultaneous vusers.
For example in a Yahoo Mail application: Suppose a scenario consists of 100 vusers with 3 tasks – 1) Login, 2) Check no of unread mails 3) Logout. Vusers at 1) + 2) + 3) will be called as concurrent vusers as they are part of same scenario performing some task but if have set a rendezvous point so that say 25 vuser perform the 2) task at the same time these 25 vusers would be termed as simultaneous vusers.