1 is the single letter i. Using short variable names such as this can result in your ending up with much shorter code and involves a lot less typing. The down side is that short names like this are not very descriptive of what the variable contains. The use of names this short is therefore best avoided under most circumstances.
2 an_apple is an example of one of the ways that you can incorporate multiple words into a variable name so as to make it more descriptive and to avoid conflicts with the reserved words. The practice of separating the words in variable names with underscores is not so popular now as it used to be.
The preferred way these days for using multiple words in a variable name is what is known as camelCase where the first letter of each word is a capital and the rest of the word is in lowercase. 3 numberOfMonkeysThatFitInABarrel demonstrates both the use of camelCase and also shows that if we want to we can use really long variable names. This one is actually longer than it really needs to be since the 'numberOf' on the front doesn't really add anything useful in terms of working out what the variable is likely to contain and so calling that field >monkeysThatFitInABarrel would be a better name to use.
4 and 5 show how we can also validly use $ and numbers in our variable names as well as letters and underscores.
This article written by Stephen Chapman, Felgall Pty Ltd.