Learn German: Die Birne ist Obst.

In German, just like in English, we drop the article when talking about uncountable nouns or broad categories. Since Obst refers to fruit in a general, collective sense, we just say ist Obst instead of using the word ein.

What is the difference between Obst and Frucht?

Obst is a collective word for fruit as a type of food or produce (like apples, pears, and berries). Frucht refers to a single piece of fruit or the botanical fruit of a plant. When categorizing food, like saying a pear is a type of fruit, always use Obst.

Does Obst have a plural form if I am talking about many fruits?

No, Obst is an uncountable noun and does not have a plural form. Whether you have one pear or a whole basket of different fruits, it is all just Obst. If you specifically need to count different kinds of fruit, you would use a word like Obstsorten (types of fruit).