The verb "to be" is translated by two verbs in Spanish: ser and estar.

Basically the verb ser is used:

• with an adjective to denote a constant feature: ella es bonita - she is pretty;
• always with a noun, numeral, pronoun (questions: who is it? how much is it? what is it?): él es médico - he is a doctor, Juan es médico;
• to specify the time (son las tres - it's three o'clock);
• in a construction with "de" to indicate the origin or material: esto es de hierro - this is of iron;
• see: passive .

The verb estar is used:

• in response to the question: where ?, eg Madrid está en España - Madrid is in Spain;
• with an adjective to denote a temporary, transient feature: estoy enfermo - I am sick;
• with a gerund in a special construction: estoy escribiendo - I am writing.

Both ser and estar are irregular verbs. You can click the link to see their conjugation.