As my colleagues have said in previous posts, "ser" and "estar" are both forms of the verb "to be". These verbs are influenced in some ways by time (hours or verb tenses), location, and permanence. The difference is that "ser" can be used to describe people and things in terms of their permanent qualities and by using adjectives that qualify and quantify the noun. For example, if you are describing a flower as a red rose, you would say "La flor es una rosa roja". In this case "es" is a form of "ser". Notice it is placed after the noun you are describing (flor) and before the quantity and adjective (una rosa roja). Since the noun is not an object that is interchangeable, meaning it is a flower and it will always be a flower, then we use the "ser" form.
On the other hand, if you are describing qualities that are conditional, conjugating a verb in the progressive tense (-ing endings, raining, running, etc.), or talking about location, then you would use the "estar" form of the verb "to be". For example, "I am writing a letter. It is raining outside, so I am home." In Spanish you would say "Estoy escribiendo una carta. Esta lloviendo afuera, entonces estoy en casa".
Although it sounds confusing at first, keeping in mind the concept of permanence and lots of practice helps in deciding when to use "ser" or "estar". I usually recommend that my students listen to or watch Spanish TV to get used to the conversational ways verbs can be used. Also, increase reading in your target language to be more familiar with the grammatical forms of the verbs.
Hope this helps! Buena suerte!