Updated: Nov 27, 2020
各 is more like "each".
It is more frequently used when the intention is to describe some sort of individuality of some group. It is more common to see this character in describing the difference in each item in a group.
Example: 各种不同的花 (many different kinds of flower).
每 is more like "every".
It is used more when the intention is to describe a group's collective property that all members share, and consequently seen more often when describing the sameness of items in a group.
Example: 每种花都一样 (all the flowers are of the same kind).
The confusion is that they are both used when describing some property of the individual in a group, but with different intentions. You can see them as two kinds of "every" with different connotations.
When used as 'every', sometimes, they are interchangeable, e.g. (每/各)个学校. But:
With some nouns, 每 can be used directly without quantifier but 各 can't. E.g.:
每星期 -- good 各星期 -- weird
But sometimes it's is reversed.
每单位 -- wrong 各单位 -- good
In this case, 每 must be followed by quantifiers: 每个单位 -- good
Generally speaking, 每 is usually followed by a quantifier. Those not are exceptions.
每 can be followed by all kinds of quantifiers while 各 can be followed by limited quantifiers. In this sense, 每 is more widely used compared to 各. E.g.:
各件商品 -- wrong 每件商品 -- good
各个学校 -- good 每个学校 -- good
(各/每)位领导 -- good, but used in different situations
充斥着各种声音 -- good 充斥着每种声音 -- doesn't make sense
各个大学之间的区别 -- hrm ... a little uneasy 每个大学之间的区别 -- doesn't make sense
每 can be followed by numeral while 各 never (when used as 'every').
每五人 -- good 各五人 -- good such in 男女各五人 but it does not mean 'every'