Children who draw and scribble are learning that they can represent their ideas on paper, both with pictures and abstract symbols. Children in these two stages are developing fine-motor control. Parents and teachers can help pre-literate writers by listening as they describe their pictures or read their scribbles, and then writing simple words to reflect these descriptions.
When children reach the stage of producing letter-like forms and random letters, they are in an emergent stage of writing. They understand that abstract shapes can carry meaning beyond the pictures that they draw. Children in this stage often pretend to write by stringing together letters and shapes without any regard for spacing or sizing. These children don't realize that specific sounds are represented by specific letters, but they will happily read a "story" that they have written. Children in this stage often reverse letters or form them incorrectly.
In this stage of writing, children learn that words represent thoughts and ideas, not just scribbles and abstract shapes. They "invent" their spellings by writing letters to represent the sounds that they hear in words. Often, children hear only the first sound in a word, so one letter may represent an entire syllable or word. As children learn more about which letters represent which sounds, their spelling becomes more accurate. Spacing and punctuation also start to appear at this stage, but it's often inconsistent. This stage is typically called the "phonetic stage" of writing, because children write down the sounds that they hear in words.
This is the final stage of writing development. In this stage, most words are spelled correctly, and spacing and punctuation are also generally correct. A child in this stage of writing will be able to write several sentences that communicate an idea, and a person who does not know the child will be able to read the writing with minimal difficulty. This stage resembles adult writing, even though there may still be the occasional spelling or grammatical error. At this stage, most children no longer reverse individual letters.