Written and spoken language differ in many ways. However some forms of writing are closer to speech than others, and vice versa. Below are some of the ways in which these two forms of language differ:
Writing is usually permanent and written texts cannot usually be changed once they have been printed/written out.
Speech is usually transient, unless recorded, and speakers can correct themselves and change their utterances as they go along.
A written text can communicate across time and space for as long as the particular language and writing system is still understood.
Speech is usually used for immediate interactions.
Written language tends to be more complex and intricate than speech with longer sentences and many subordinate clauses. The punctuation and layout of written texts also have no spoken equivalent. However some forms of written language, such as instant messages and email, are closer to spoken language.
Spoken language tends to be full of repetitions, incomplete sentences, corrections and interruptions, with the exception of formal speeches and other scripted forms of speech, such as news reports and scripts for plays and films.
Writers receive no immediate feedback from their readers, except in computer-based communication. Therefore they cannot rely on context to clarify things so there is more need to explain things clearly and unambiguously than in speech, except in written correspondence between people who know one another well.
Speech is usually a dynamic interaction between two or more people. Context and shared knowledge play a major role, so it is possible to leave much unsaid or indirectly implied.
Writers can make use of punctuation, headings, layout, colours and other graphical effects in their written texts. Such things are not available in speech
Speech can use timing, tone, volume, and timbre to add emotional context.
Written material can be read repeatedly and closely analysed, and notes can be made on the writing surface. Only recorded speech can be used in this way.
Some grammatical constructions are only used in writing, as are some kinds of vocabulary, such as some complex chemical and legal terms.
Some types of vocabulary are used only or mainly in speech. These include slang expressions, and tags like y'know, like, etc.