What is the difference between Multiple robots and swarm robots? What is the key point? Also what is multi agent systems? Do multi agent systems works only for computer simulations or games? These terms are used similar applications.
These concepts are similar in the sense that they require multiple robots that communicate/cooperate. Apart from that, their application, and thus their design and implementation differ.
Swarm robots are designed after ants, bees and such creatures. The idea therefore is not just about multiple robots cooperating, but it's about many robots, each of which is very simple. With swarm robots, you would expect each robot to be too stupid to possibly even know why it's doing something (i.e., what is the ultimate goal). Their simple function however, would result in a complex algorithm when a bunch of robots are doing it.
To recap, with swarm robots, each robot is unable to do anything meaningful. The desired behavior emerges only when there are many of them. Usually, the robots all look the same.
In multi-agent systems, you have multiple agents, which could be either computer programs or robots, each of which is able to do some meaningful part of a task. It is possible that without having all the agents, the ultimate goal cannot be reached, but parts of it perhaps could still be done. Think of multi-agent systems as a society of humans. Each of us have a profession and we thrive together, but we are not useless on our own.
To recap, a multi-agent system consists of entities (e.g., robots) that are each specialized for a certain task. They cooperate to achieve the ultimate goal, but individually they are also able to do some things.
Multiple Unrelated Robots
You mentioned multiple robots. That's not something that relates to a specific methodology (as opposed to e.g., swarm robots). I'm going to thus talk about multiple unrelated robots.
It is not unthinkable to imagine unrelated robots cooperating at some point in time. In such a case, each robot has its own goal, or is part of its own swarm, or is a member of its own multi-agent system, etc. Based on circumstances, the robots may meet and they may interact. One simple case is the robots avoiding crash while each goes on about its business.
In this case, there is really no common algorithm, design, methodology or whatever. It's just individual robots who happen to meet.
In addition to @Shahbaz
According to this book
Multiple Mobile Robot Systems is main topic and swarm robotics is a sub topic both of them motivated from
the task complexity is too high for a single robot to accomplish
the task is inherently distributed
building several resource-bounded robots is much easier than having a single powerful robot
multiple robots can solve problems faster using parallelism
the introduction of multiple robots increases robustness through redundancy
Swarm Robots: Historically, some of the earliest work in multirobot systems dealt with large numbers of homogeneous robots, called swarms.
This means multi robot systems can be homogeneous and heteregeneous but swarm systems have to homegeneous.
Multi agent systems is a computerized systems composed of multiple interacting intelligent agents within an environment. But also it can be robot.