What is imagery in literature?
It is using your words in a way that a picture or an 'image' is conjured up in the reader's mind; it's making him experience what you are talking about only through way of words.
Imagery is very often then descriptive language that allows you to 'feel' the situation rather than just go through it. It works with the senses and can deal with all of them; the olfactory, the visual, the oral, the tactile, the auditory, the gustatory etc.
Imagery exists in more than two types I think. Employing every other sense in it is another kind of imagery, and the senses I have mentioned above. Figures of speech and other poetic techniques such as alliteration, simile, metaphor, assonance, onomatopoeia etc. are all means through which imagery is created in literature.
There is a lot more left in the explanation of imagery to do suffice to it, but for that you'd better visit wikipedia, enotes or other websites of such a kind.
For a better understanding I suggest read book Fahrenheit 451or summary of books https://freebooksummary.com/fahrenheit-451-summary-2-28591 In this book many imagery such as F ire, Heat, Light and so on