*A countable Noun is one which can be counted.

* It has singular as well as plural form.

* Generally common nouns and collective nouns are countable nouns.

* It can take the articles before it e.g. a boy, a dog, an elephant, the toys etc.

* An uncountable noun is one which can not be counted.

* It doesn't have any plural form. We always use it in its singular form.

* Generally they refer to abstract and material nouns. 

* It does not take articles in front of it. But little, a little or much can be used e.g. much water, a little milk etc.

Sometimes an uncountable noun can be used as a countable one.

1. Glass:

*This bowl is made of glass.(uncountable)

* We have three glasses in our kitchen. (Countable)

2. Iron:

* Iron is a very useful material. (Uncountable)

* We use iron to smoothen our clothes. (Countable)

man, boy, girl, animal, book, river, mango, bus, car, tree, student, teacher, baby, bank, bed, cat, chair, city, class, cup, dog, door, ear, egg, hand, leg, fan, father, kite, King, lion, newspaper, page, shirt, street, table, town, month, week, minute, woman, year, pillow, zebra, axe etc.

air, water, death, food, milk, glue, oil, faith, rain, truth, honesty, beauty, poverty, power, happiness, courage, boyhood, glass, sand, hair, earth, sugar, snow, peace, failure, advice, gold, salt, gas, education, childhood, news, money, bread, jam, cheese, marriage, wealth etc.