Let's talk about such feature of verbs as transitivity. There are two categories of verbs in Russian - transitive and intransitive. Verbs of the first category are used with an object, usually it is a noun or pronoun in accusative or genitive case and the combination can be made without a preposition. If you cannot add an object (it is also called an object) to a verb, then the verb is considered to be intransitive. Intransitive verbs can denote a person's condition, position and motion in space, action and development.
The example of transitive verb with a noun in accusative case: Девочка гладит кошку - A girl is stroking a cat.
The example of intransitive verb: Кошка спит - A cat is sleeping.
The example of transitive verb with a noun in genitive case: а) meaning negation: Я не купила молока - I didn't buy milk.
б) indicating an object: Хочется почитать хороших книг - I'd like to read interesting books.
There are also such verbs which can be as transitive as intransitive depending on a situation. These verbs are called labile, there is not so great number of them in Russian. Автор пишет книгу. Автор пишет. - An author writes a book. An author writes.
---
How to define whether a verb is transitive or intransitive?
1. If a verb with a noun in accusative case requires a preposition, it is an intransitive verb.
Сесть в автобус - Get on the bus.
2. All reflexive verbs are referred to intransitive ones.
Мыться, удивляться - to wash oneself, to be surprised.
Knowledge on category of verb transitivity can help you in declension of object (a noun or pronoun) because one of the most common mistakes made by Russian-speaking beginners is the usage of noun in genitive case in the combination with intransitive verbs.