We have heard and read lot of stories involving Yakshas like Kuber was a Yaksha, there was an encounter of Yudhisthir with Yaksha who asked seven questions to him(ofcourse that was revealed to be Dharmaraj later). My question is who are Yakshas and how were they different from normal human beings. Do they possess any powers. Do they have any responsibilities towards anything? Also how come Kuber be a Yaksha when his father sage Vishrawa was a Brahmin?
Yakshas and Rakshasas are counterpart same as how Devas and Daityas are.
prajāpatiḥ purā sṛṣṭvā apaḥ salilasaṃbhavaḥ
tāsāṃ gopāyano sattvān asṛjat padmasaṃbhavaḥ
te sattvāḥ sattvakartāraṃ vinītavad upasthitāḥ
kiṃ kurma iti bhāṣantaḥ kṣutpipāsā bhayārditāḥ
prajāpatis tu tāny āha sattvāhi prahasann iva
ābhāṣya vācā yatnena rakṣadhvam iti mānadaḥ
rakṣāma iti tatrānyair yakṣāmeti tathāparaiḥ
bhuṅkṣitābhuṅkṣitair uktas tatas tān āha bhūtakṛt
rakṣāma iti yair uktaṃ rākṣasās te bhavantu vaḥ
yakṣāma iti yair uktaṃ te vai yakṣā bhavantu vaḥ
The lotus born one [Lord Bhrahma] generated creatures for protecting that element (water). And thereupon those creatures humbly presented themselves before the creator, saying, What shall we do? We are sore tried by hunger and thirst. The lord of creatures, laughing, addressed them, saying, Ye men, do ye carefully guard this (water). Thereat some said, "Rakshama", and others, "Yakshama".
Thus accosted by those afflicted with hunger and thirst, the creator said, Those among you who have said Rakshama, shall be called Rakshasas and those among you who have said Yakshama shall be called Yakshas.
Yakshas' lineage is not mentioned clearly in scriptures but they engaged in virtuous deeds, became powerful and came to be known as demigods.
Later Vaisravana (son of Visrava i.e. Kubera) did intense penance and got boons from Lord Bhrahma of protecting people (the post Lokapala) and god of Riches (the post of Vitarakshanam). Lord Bhrahma gave him Pushpaka vimana too and provided the status equivalent to Devas.
- Kubera became king of Yakshas and Lanka.
On another hand Rakshasas too did intense penance, became powerful but didn't engaged in virtuous deeds.
Among the Rakshasas, who thus guarded the sacred waters, Hoti (or Heti) and Prahoti became the leaders of the Rakshasas. Praheti went in forest for tapsya.
But Heti wanted to marry and got married to Kala's sister Bhaya. Heti's son was Vidhyutkesha. Sukesha (who was abandoned by his mother soon after his birth but later was blessed by Lord Shiva and Paravati) was son of Vidhyutkesha. Sukesha's sons were Malyavan, Sumali and Mali.
Malyavan's children: Vajramushti, Virupaksha, Durmukh, Suptaghana, Yaghyakopa, Matta, Unamnta, Analaa
Sumali's children: Prahasta, Akampana, Viakata, Kalikamukha, Dhumraksha, Danda, Suparshva, Sanghadi, Praghasa, Bhaskarna, Kumbhinashi, Kaikashi (Ravana's mother), Rakaa, Pushpotaktaa
Mali's children: Anala, Anila, Hara, Sampati (later they became ministers of Vibhishana)
Lanka was built by Vishvakarma. Malyavan, Sumali and Mali used to live in Lanka. Later they attacked Devas and Lord Vishnu came to fight with them. Lord Vishnu killed Mali. Seeing Mali slain, Sumali and Malyavan, burning in grief, led precipitately towards Lanka along with their forces. But Lord Vishnu didn't stop killing Rakshas.
- Malyavan, Sumali and Mali became king of Rakshasas and Lanka.
And albeit inlated with pride, they, incapable of coping with Vishnu, leaving Lanka, went to the nether regions (Rasatala) with their wives for dwelling there. Then Kubera started living in Lanka.
- Kubera became king of Yakshas and Lanka.
Later Sumali encouraged his daughter Kaikesi to marry Visrava. Coming to know that his daughter's sons, Dasanana and his two brothers, got powerful boons (Dasanana was incapable of being slain by Nagas, Yakshas, Daityas, Danavas and the Devas) form Lord Brahma, Sumali shed all his fear of Vishnu and came out from Rasatala. Later Sumali encouraged Ravana to take Lanka back from Kubera. Vishrva advised Kubera to leave Lanka and build another city name Alkapuri on Kailash mountain which Kubera accepted.
- Ravana became king of Rakshasas and Lanka.
How are they different from normal human beings, do they possess any powers? How come Kuber be a Yaksha when his father sage Vishrawa was a Brahmin?
Yes, they possess powers. Nature of Rakshas' power is Tamsik however nature of Yakshas' power is Rajasik. Kubera didn't born from Yakshas, his grand father was a Bhrahmrishi and his father was Visharva and mother was a daughter of Rajrishi. But later he evaluated to the king of Yakshas. As I mentioned above Kubera did intense penance and asked boons from Lord Bhrahma and became Lokapala, God of riches and king of Yakshas. On another hand, Ravana's father was also same Visharva (however mother was different) but Ravana joined his maternal uncles who were from Rakashas. Rakashas later appointed Ravana as their king.
Valmiki Ramayana: UTTARA KANDA: Sarga 3 - Sarga 11
Yaksha is very similar to Jinn (or Ginny). They are usually demonic entities (as per ISKCON) or Ogres (as per Gambhirananda). Though they are similar, they are different from the Rakshasa.
From Wikipedia article:
Yaksha (Sanskrit yakṣa, Pali yakkha) is the name of a broad class of nature-spirits (or demons), usually benevolent, who are caretakers of the natural treasures hidden in the earth and tree roots. They appear in Hindu, Jain and Buddhist texts. The feminine form of the word is yakṣī or Yakshini (yakṣiṇī).
In Hindu, Jain, and Buddhist texts, the yakṣa has a dual personality. On the one hand, a yakṣa may be an inoffensive nature-fairy, associated with woods and mountains; but there is also a darker version of the yakṣa, which is a kind of ghost (bhuta) that haunts the wilderness and waylays and devours travelers, similar to the rakṣasas.
According to Gita, the people with Rajasic nature worship them.
यजन्ते सात्त्विका देवान्यक्षरक्षांसि राजसाः। प्रेतान्भूतगणांश्चान्ये यजन्ते तामसा जनाः
BG 17.4 - Those with sattva, worship the deva-s; those having rajas, worship the ogres (yaksha) and monsters (rAkshasa); Other people possessed of tamas, worship ghosts and the spirits