Parallel Vs. Grid Vs. Distributed Vs. Cloud Computing

Modern days network-connected users thrive for far better computing power. The computing power is a combination of Hardware, Software, and networks.

Here are four kinds of connected models. Those are

1.Parallel Computing

Now a days jokingly in the software development, people follow this processing. But it s actaully agile not paralall.
In this type of computing, all the computers keep in a single room. These systems share their work and finish.
The best example is PARM computer India.
Another best computer is Intel Paragon.
Sources: Algorithms for Parallel Processing: 105 (The IMA Volumes in Mathematics and its Applications)

Grid means not in one place. In INDIA, Power Grid performs a common task, which supplies power. Exact way, Grid-connected computers are located at distant geographical locations and work towards a single-task.
Sources: Grid Computing: Techniques and Applications (Chapman & Hall/CRC Computational Science)
A central processing unit controls the grid and gets computing-power to deliver the task. The high point is, the Grid resources are heterogeneous (that means not similar).
SETI Grid-connected Computer connects to idle-PCs all over the world and uses this power as a supercomputer.

More or less meaning is similar to Grid computing. But actually, it is not. The connected powers in a Distributed model are called NODES. All the nodes work towards a common goal.
Sources: Distributed Computing: Fundamentals, Simulations and Advanced Topics, 2ed

It is the 21st century, and working and delivering task is past now. With the virtualization takes place, the physical-resources can virtualize as needed.
The Prime goal is reduce cost, and quicker delivery.
Resources consumption is on-demand. I need X computing-power then I will get X computing power. The other guy needs Y, and he gets Y. In general, it is elastic. It expands and shrinks according to demand – this is the power of the cloud.
Cloud is a combination of Hardware, Software and Network. All these resources you can get on-demand.
Nowadays, you can see, Public, Private and Hybrid models of the Cloud. Private means in the premises. Public means outside premises. Hybrid means a combination of these.