Cohesion concerns how explicit cues are used in a text to signal relations in that text. These cues are known as cohesive devices and they include reference Go to the Reference page, substitution Go to the Substitution page, ellipsis Go to the Ellipsis page, conjunction Go to the Adverbials page, and lexical cohesion Go to the Anaphoric Nouns. page. Cohesive devices and words themselves are often repeated throughout a text creating a rich network of cohesion. You can see how this works on the lexical chains Go to the Lexical Chains. page page.
Coherence is not a textual feature. A text is coherent if it corresponds to accepted norms in such a way that a reader feels that it follows a logical flow.
Cohesive devices help (or should help) produce a coherent text. But the extent to which a text is coherent is a decision for the reader. Different readers will make different decisions about the coherence of a text based on their own linguistic skill and their knowledge of the subject matter of the text. This one reason why a writer must bear in mind the expected audienceYour expected readers. See the glossary definition for text.
Good writers use cohesive devices with skill and flexibility. They understand when it is better to use them in order to make their meaning clear and when not to to use them (because a competent reader would be able to make the connections without their aid).
There is no direct relationship between the quality of your writing and the number of cohesive devices used. Students should be discouraged from peppering their texts with cohesive devices, especially adverbials Go to the Adverbials page, just in order to show they can use them.