Can anyone explain to me precisely what the technical difference is?
Well, a conjunction, as the name implies, is used to join two parts of a sentence. They are words like but, and, or, that, etc.
e.g. He said that he likes Jill.
e.g. He likes Jill because she’s sweet.
A preposition is word placed before a noun or noun phrase and somehow indicates the noun or noun phrase’s relationship to another verb, adjective or noun (phrase.) These are words like over, by, in, to, with, etc.
e.g. Jill climbed over the wall.
e.g. Jill stood near the bus stop.
As you can see, prepositions establish a relationship between two parts of a sentence in a way that conjunctions don’t. Conjunctions act more like glue, if you will, and don’t add a lot of meaning. Prepositions usually establish some sort of positional relationship between elements in a sentence.
I seem to remember when I was a kid learing about such things and the teacher presented each of us with drawing of a person standing next to a tree. She told us to think about how the person could get to the other side of the tree or how the person relates to the tree. (The person could go over, under, through, near, by, on, etc) That’s how I identify a preposition.
And as for a conjunction, sadly I remember the “Conjunction Junction, what’s your function? Hookin’ up words and phrases and clauses.” from Schoolhouse Rock.
As has been noted, conjunctions join things. These could be sentences (called clauses when they are combined into a bigger sentence), or just items.
The coordinate conjunctions join things, but show little or no relationship between clauses:
The boy kicked the ball. He scored a goal.
becomes
The boy kicked the ball and he scored a goal.
There are six coordinate conjunctions (for, and, nor, but, or, yet), though some add so, for a total of seven.
Subordinate conjunctions join clauses, and show a relationship between them:
I was waiting for the bus. I saw Bob.
can become
While I was waiting for the bus, I saw Bob.
or
I saw Bob while I was waiting for the bus.
Another example:
The bus was late. I was late for school.
can become
Since the bus was late, I was late for school.
or
I was late for school because the bus was late.
Prepositions give direction to actions. They are not adverbs, though they do add information to what is happening in the sentence:
I sat on the chair.
Bob saw me at school.
The car is driving over the bridge.
Susan goes to work in the city.
John rode on the train to Chicago in December.
The difference between conjunctions and prepositions is just that conjunctions join things, while (hey! I used a subordinate conjunction there) prepositions give direction to actions. Here’s an overly simple test, but (coordinate conjunction) it works in (preposition) most instances:
If you take the word you are wondering about out and end up with two sentences, you have a conjunction. If you take that word out and end up with an incomplete sentence somewhere, you have a preposition:
Remember:
The boy kicked the ball and he scored a goal.
The boy kicked the ball. He scored a goal.
Take away and, and we have two sentences. And must be a conjunction.
But:
Bob saw me at school.
Bob saw me. School.
Take away at, and we have one sentence and only part of another. At must be a preposition.
The test won’t work in all instances. But it works in many, and demonstrates the difference between the two parts of speech.
Whoops, forgot something.
Conjunctions can join things to make plurals, and prepositions can still be used. The test won’t work so clearly at those times:
I saw Steve and Mary at the movies.
becomes
I saw Steve. Mary at the movies.
(Removing the conjunction doesn’t leave us with two sentences, so you might mistake it for a preposition.)
I saw Steve and Mary. The movies.
(It works here, so the removed word must be a preposition.)
Like I said above, the test won’t always work. Be aware of situations like the example in this post however, and you should be okay.
Some people get confused between a preposition and an adverb when the same word can be used for both. “The game is over.” That sentence does not end with a preposition (although that is allowed), but by an adverb. “Drink up,” is similar, describing how you should drink. There are countless others: “Please have John fill in,” etc.
Actually, the rule that one does not end a sentence with a preposition is not, linguistically speaking, a rule of English at all. It is a descriptive (as opposed to a prescriptive, or “formal”) rule of Latin, which was imposed as a prescriptive rule in English, much like the rule about split infinitives. It does not work in English because of our plethora of verb phrases composed of a verb + a preposition.
thanks! I finally get it.
Now if I can just get to grips with the difference between adverbial phrases and adjectival phrases…
An adverbial phrase does the job of an adverb. It’s a group of words that describe a verb.
Johan knitted as if his life depended on it
Johan knitted quickly
An adjectival phrase does the job of an adjective. It’s a group of words that describe a noun.
Christine likes programmes about ancient history
Christine likes educational programmes
Now that we’re doing your English homework for you, please post your grade so we’ll know what we got.
Hey Barbitu8
we done real good! 
It’s not my English homework by the way. I asked the teeming millions because I was genuinely confused and every time I looked at my text books, it was too complicated. I’m doing this at a tertiary level and I am the result of an education system which gave up on teaching grammar at all. It’s not a pretty sight. Sitting in a class with a student who swears black and blue that ‘apple’ is a verb is a bit of a worry.
Here is a mnemonic for remembering the conjunctions for, and, nor, but, or, yet, so:
Taking the first letter of each word:
FANBOYS.