Hinduism is quite an interesting practice; however, for someone who is new to it, it has a lot of things to offer to make it appear complex and mystical. 

Among all these practices, Isht Devta (personal deity), Kuldevta/Kuldevi (family deity), and Devta (deity) are some of the more significant ones in the devotional life of Hindus. 

The reason these terms lie apart in a way is that they form different layers of Hindu worship for personal spiritual journeys that, of course, in many instances, have remained tradition-based for the family.

Isht Devta : Your Personal Deity The word Isht Devta consists of much lesser Sanskrit words: Isht, meaning desired or crushed, and Devta, meaning deity. Thus, Isht Devta is the personal deity that an individual selects according to their spiritual inclination. Each journey is unique in this regard-that an individual's way is not like that of another but stands as a unique model.

About this, in Bhagavad Gita (Chapter 7 Verse 21), Lord Krishna has said: "For whatever form of deity the devotee who has faith wishes to worship, I stabilize that person's faith in that form."  

This shows that according to Hinduism, the Divine can be approached through many forms so that everyone may have contact within personal resonance with that manifestation. Another aspect of the Padma Purana is that the devotees should select their Isht Devata according to one's inclination and devotion rather than according to any impositions or conventions.

Kuldevta/Kuldevi: The Familial Deity 

Worshipped across generations, Kuldevta (male god) or Kuldevi (female god) is the family god or clan god.. Kul means family or clan, and through this attachment, family members are linked to each other with respect to a particular spiritual lineage owing to this devotional practice.   

This is very explicitly discussed in the Skanda Purana since there are many references to Kuldevta there and also his importance in taping the family-arising lineages, with many Hindu families having a space in their homes dedicated to the Kuldevta/Kuldevi, where regular offering and prayer takes place.

Specifically, it is mentioned in Dharmashastra texts, such as Manu Smriti, Chapter 3, that family and such traditions should maintain these to keep prosperity and spiritual well-being of the family line in all times.  

Devta: Divine Beings

Devta refers to any divine being in the broadest sense in Hinduism. Rigveda, among the oldest texts of Hinduism, mentions 33 core Devtas, but Hinduism recognizes innumerable divine manifestations.

"This many forms really stand for the one supreme reality, Brahman," comments the author of the Brihadaranyaka Upanishad(3.9.1):

 "Three hundred and three, and three thousand and three," he answered. 'Yeah,' he added. "but how many gods are there, Yajnavalkya? " Thirty-three. " 0 "Yes," said Yaynavalkya, "but how many gods are there? Six." "Three," he said, "Yes, but how many gods are there, Yajnavalkya?" "Yes," he said, "but how many deities are there, Yajnavalkya? "Two.μ "Yes, but how many gods are there, Yajnavalkya? "One and a half. per day" "One god is there, Yajnavalkya? " "Yes," he said.

How These Concepts Work Together 

Thus in actuality, these concepts form concentric circles of devotion:  

Your Isht Devta represents your own personal choice in the spiritual journey 

Your Kuldevta/Kuldevi connects you to the divine heritage of the family 

Broader pantheon of Devtas represents the diversity of divinity   

For one new to Hinduism, understanding these things gives a starting point toward grappling with the rather complicated seeming structure of Hindu worship. You might start by trying to learn about different gods to find an Isht Devta that fits your spiritual temperament, while also being glad to be part of the larger traditions. 

The Vishnu Purana (Book 1, Chapter 9) gives this approach such a beautiful summary: 

"As a will-of-the-wisp ultimately falls into the ocean, so, worship offered to any deity will ultimately lead to the Supreme." 

In this regard, most centers of modern Hindu spirituality have inculcated such broader spiritually engaged paths, like Swami Vivekananda, in his work "Raja Yoga," that advise to individual realization of the inherent divinity and ultimate reality.