An array is a series of elements of the same type. It can have varying lengths (An analogy would be a building which can have many floors, but all the floors would be designed in the same way).
A cluster is a collection of elements which can be of different types, like a bag - it can have an apple, three oranges and a cell phone, but you carry them in a single bag.
A string is simply a sequence of alphanumeric characters, like a word or a sentence.
This is all really basic stuff. To learn more about LabVIEW, I suggest you try searching this site and google for LabVIEW tutorials. Here, here, here and here are a few you can start with and here are some tutorial videos. You can also contact your local NI office and join one of their courses.
In addition, I suggest you read the LabVIEW style guide and the LabVIEW user manual (Help>>Search the LabVIEW Bookshelf).
___________________
Try to take over the world!
