What are "coherence" and "cohesion" in text linguistics?

I am still learning English. My English language professor has given me an assignment on coherence and cohesion. But it seems difficult to me. I've consulted my friend and he told me:

Cohesion and coherence are terms used in discourse analysis and text linguistics to describe the properties of written texts.

A text may be cohesive without necessarily being coherent: Cohesion does not spawn coherence. Cohesion is determined by lexically and grammatically overt intersentential relationships, whereas coherence is based on semantic relationships.

Phew too difficult! Can anyone please explain the meanings, differences and examples of these to me, in simple and easy English?

Cohesion describes the way in which a text is tied together by linguistic devices, such as And so we see . . . , Additonally . . . , Therefore . . . , However . . . and On the other hand . . .

A text has coherence if its constituent sentences follow on one from the other in an orderly fashion so that the reader can make sense of the entire text.

Cohesion is "the glue that sticks a sentence to another in a paragraph or a paragraph to another in a text."

A text can be cohesive through the use of the following devices:

1. Repetition. In sentence B (the second of any two sentences), repeat a word from sentence A.
2. Synonymy. If direct repetition is too obvious, use a synonym of the word you wish to repeat. This strategy is call 'elegant variation.'
3. Antonymy. Using the 'opposite' word, an antonym, can also create sentence cohesion, since in language antonyms actually share more elements of meaning than you might imagine.
4. Parallelism. Repeat a sentence structure. This technique is the oldest, most overlooked, but probably the most elegant method of creating cohesion.
5. Transitions. Use a conjunction or conjunctive adverb to link sentences with particular logical relationships. There are many kinds of transitions.

Coherence means that the text is easy to read and understand because the text follows a certain kind of logical order and the organization of ideas is systematical and logical.

Some kinds of logical order:

chronological order, spatial order, order of importance

The answers here are good, but examples are missing. Example of a coherent text that is not cohesive:

Summer was over. The boy went to school. The building: Peter had never liked it. All the other class members became easy targets of the lawmaker's son's gun. At 8:15 the massacre began. 7 children would not go home. The last words of the juvenile perpetrator: "I hate Mondays".

Example of a cohesive text that is not coherent:

You may not fully understand the reasons underlying the incident, so let me try to explain. Namely, it may help your understanding to know that Peter had not been happy about the oranges, after all. Nor was the other Peter ever again able to fly after the aforementioned fruity explanation. Then, weeks later, he realised that it was because my fruit and wings rationale is lacking in substance. If that is all, you may find it hard to get the point of what I'm driving at. Nevertheless, I think it was worth trying to explain.

As should be clear from these examples, coherence without cohesion can be an effective literary technique and in fact is often used that way. Whereas cohesion without coherence is just weird and confusing, and is of very limited use in literature.

Coherence concerns text unity whereas cohesion deals with sentence unity.