camelCase

The most ubiquitous case style in the world of programming is the camel case.

Imagine this: You are writing in lowercase and you have to type a group of words as a single keyword. You can't use space as it is used to separate keywords. You also can't use symbols. What do you do?

Here's an idea: What if you use capitalization, not as emphasis as it is usually used for, but as word separator.

justLikeThis

Problem solved.

Since programmers are often bound to the same constraints mentioned above, especially when naming variables and function names, it's easy to imagine why programmers use it a lot, especially for variables and function names.

function calculateAreaOfCircle(radius) { ... }

PascalCase

A variation of the camel case except the first letter is also capitalized. It sort of has the same effect as the title case. As such, it is used when the keyword needs to be distinct from the usual variable naming style, such as a class name.

class UserAccount { ... }

In programming languages like C#, it is a convention to use pascal case for variables in general. Consult your local tech lead to know when you should be using pascal case.

Acronyms

We usually uppercase acronyms in a sentence, such as URL, HTTP, etc. As such, people do so in camel case as well.

function parseURLString(url) { ... }

While it's easy to make out that URL is a single word here, you can imagine how difficult it is to make out when a word starts and ends. Especially when it's a word you are unfamiliar with. And what happens when there are two acronyms?

function getHTTPAPIMethod(request) { ... }

I like to treat the entire string as lowercased, then capitalize the beginning of each word, including acronmys. I just find it better to read that way.

function parseUrlString(url) { ... }
function getHttpApiMethod(request) { ... }

This is obviously up to personal taste. Keeping acronyms all-caps in camel case is perfectly acceptable. The only acceptable way in some teams, even. Just remember the platinum rule.