Caste System as per Bhagavad Gita

चतुर्-वर्ण्यं मया सृष्टं गुण-कर्म-विभागशः ॥ 

catur-varṇyaṃ mayā sṛṣṭaṃ guṇa-karma-vibhāgaśaḥ ॥

“The four orders of society, brahmin, kshatriya, vaishya and shudra were created by Me (God) and classified according to the qualities predominant in all and taking into account their corresponding duties.”

As per Bhagavad Gita, every human goes through four stages of life. They encompass teachers, warriors, traders and servants. There is the living presence of God in every human being. God therefore created four aspects in each human being - a shudra, a vaishya, a kshatriya, and a brahmin depending on the nature of the work they perform at a given time. 

Hindu God Shiva-Vishnu-Brahama-pay-homae-to-Mahadevi-and-Ganesha

Shudra is the one who serves others. So, when you are taking care of the body, serving the body, you are a shudra. When you become body-conscious; when you eat, dress or take a bath, you are a servant and Shudra.

You become a Vaishya, or trader when you are engaged in earning money by whatever occupation you choose, be it teaching, treating patients or trading. Therefore, everybody who is working for money or dealing with money is Vaishya.

Your body is a kingdom and you are the king or queen of that kingdom. To maintain a strong body and mind, to sustain the body’s defence mechanism, you undertake some exercises, you must become a kshatriya or warrior. One who controls and destroys inner enemies is a true hero.

Finally, as you try to meditate a little, listen to a spiritual lecture and feel inspired by a holy book you read, and help others to lead a better life, you become a brahmin, or a lover of knowledge. With knowledge we live and grow. When you study, listen to discussions, and sharpen your intellect, you are a brahmin. 

Every day you play different roles with duties of different castes at different times of the day. Every person follows different castes at different times depending upon the gunas and karma. Karma (action and its results) is always influenced by the three mahā gunas - Sattva (Purity), Rajas (Activity) and Tamas (Idleness). They in turn reflect different aspects of everyday life, a person predominantly Sattvic in nature and action is Brahmin, one with rajasic quality is kshatriya, rajasic and tamasic together makes a person vaishya and a tamasic person is a shudra.