Preposition:
* I went to the store for milk. (Indicates purpose)
* He waited for her at the bus stop. (Indicates duration or expectation)
* This gift is for you. (Indicates recipient)
* The book was written for children. (Indicates intended audience)
* She works for a large corporation. (Indicates employer)
Conjunction:
* We waited for the rain to stop, for we were going to the park. (Indicates reason)
* I went to the library, for I needed to research a paper. (Indicates reason)
* He studied hard for the test, for he wanted to get a good grade. (Indicates reason)
Other uses:
* For goodness sake, stop arguing! (Used as an exclamation)
* For all I know, he might be right. (Used to express uncertainty)
* For instance, you can use a spoon to stir the soup. (Used to introduce an example)
The word "for" is very versatile and can be used in many different ways. The specific meaning will depend on the context of the sentence.