Coherence

Coherence refers to the organization of meanings in relation to one other. Although various meaningful sequences are possible, there must be a logical relation between sentences for a text to be coherent and achieve its communicative purposes. This logical relation is what gives a text continuity of sense.
While cohesive ties are likely to differ from language to language, coherence sets up relationship that are conceptual and are seen to be universal. Examples of these ties are cause and effect, time sequences, argumentative discussion.
As reported by Renkema, two basic types of logico-semantic relations can be distinguished: the additive relation and the causal relation.
The additive relation can be traced back on a conjunction and it is related to various types of coordination, represented by words such as AND (addition), BUT (contrast), OR (disjunction).
The causal relation can be traced back to an implication.
The most important causal relations are those corresponding to the seven types distinguished in traditional grammar:

CAUSE: expresses a consequence outside the domain of volition
               John didn't go to school. He was sick.

REASON: a volitional aspect is present
                  John didn't come with us. He hates parties

MEANS: a deliberate usage of a cause in order to achieve a volitional consequence
               Would you mind opening the door? Here is the key

CONSEQUENCE:
                            John is sick. He is not going to school.

PURPOSE: expresses a  volitional consequence
                   The instruction should be printed in capital letters. So everybody pays attention

CONDITION: expresses a necessary or possible cause or reason for a possible consequence
You can get a job this summer. But first you have to pass the exam.

CONCESSION: expresses a cause or reason for which the expected consequence fails to occur.
He was rich. Yet he never gave anything to charity.


Cohesion and coherence occur at two different levels: cohesion is at the surface level while coherence involves the deep logico-semantic relations between sentences.
As Ulrych explains, the requirement for cohesion is that 'a text should be interrelated grammatically and lexically', while the requirement for coherence is that 'a text should hang together conceptually'.