Ser and estar can both be translated as "to be." But its use depends from the context:
La manzana está verde.
(condition: verde = unripe)
La manzana es verde.
(essential characteristic: verde = color green)
It's perfectly valid to say
La mesa está hecha de madera
and to say
La mesa es de madera
Both are the same. But, suppose we need to talk about the condition of the table, then we use:
La mesa es de madera y está deteriorada.
Wich mean the table is made of wood and is damaged.
Ser is used with:
Elements pertinent to your or others' identity
Physical description, personality and character, nationality, race, gender, profession, origen, What things are made of
Things which "Take Place" or "Occur" in Time:
Dates, days, seasons, time, events, concerts, parties
Possession
Estar is used with:
Emotional, physical & mental states of (our bodies') being:
Feelings/moods/emotions, physical conditions or appearances, civil state (married, single, divorced, dead)
Placement State of Being:
Location of things and people (but not events)
Motion State of Being