When it comes to letters, case refers to whether letters are written in larger uppercase form, which is also often known as majuscule or capital letters, or smaller lowercase form, which is also known as miniscule or small letters.

Is it better to type in uppercase or lowercase?

It's faster and easier to type in lowercase, so when it's valuable to be quick and natural (or to seem that way), like on IM, lowercase is often the default. Messages typed all in lowercase can feel more offhand. The stakes feel lower, the vibe is calm.

Should I teach lowercase or uppercase first?

The upper case letters have a special, important job and should be taught in context. They show the start of a sentence or a name. 'Capitals' are best taught initially as the first letter in a child's name.