When we write, we normally use complete sentences. We use appropriate punctuation and we expect grammar, spelling and vocabulary to be more correct than in speaking.
In speech, we can say things like Forecast’s bad. – using contractions and missing out words that we don’t need. (ellipsis).
When we speak informally, we don’t have to be so careful with grammar. For example, many speakers say things like: There’s two people at the door. In speech even though they write There are two people at the door.
When we speak we use spoken tricks to keep the conversation going and to organise what we are saying.
When we write we can use punctuation such as question marks and exclamation marks to show emotion and grammatical functions