Normally Pralaya means "finishing everything".

According to Vishnu Puraan, 5/2, for every being Pralaya is of three types - Naimittik, Praakrit, and Aatyantik Pralaya. Among them ---

(1) Naimittik (or Braahm) Pralaya
Occurs in the end of the Kalp. When 1,000 Chatur-Yug are over, Prithvi deteriorates. There is no rain for 100 years. At that time all weaker living beings die. Then Vishnu makes the Prithvi completely empty by merging all of its water and all of its living beings in Himself. At that time influenced by Bhagavaan and strengthened by that water, Soorya's seven rays become seven Soorya. All seven Soorya continuously burn all Tri-Lok and Prithvi becomes very hard like the back of the tortoise. Then Shree Hari appears from Shesh Naag's mouth in the form of fire and starts burning Paataal Lok from bottom (read the location of all Lok here). That fire reaches Prithvi after burning seven lower Lok (from Paataal upward) and then burns Prithvi also; then it proceeds towards Bhuvar Lok and Swarg Lok, and after burning them that fire starts revolving at the same place. Thus after being burnt, the whole Tri-Lok looks like a red hot disc. At that time the habitation of Bhuvar Lok and Swarg Lok move to Mahar Lok, but they cannot live there because of the intense heat, so they finally move to Jan Lok. After this Vishnu produces Samvartak named multi-colored and multi-sized clouds form His mouth air and pacifies the fire. Then there is water all around. The clouds cover even those Lok which are above Bhuvar Lok. They rain for more than 100 years. Hey Maitreya, This is what happens in the end of the Kalp."

(2) Praakrit Pralaya
Because of no rains, when all Lok are destroyed and all Vikaar (from Mahat Tattwa to Vishesh) get deteriorated then water absorbs Prithvi's attribute smell. When Prithvi's smell is gone, Prithvi is destroyed. At that time water start flowing all around with a great force. Then fire absorbs water's attribute Ras; as Ras is drawn from water, water is also destroyed and takes the form of Agni (fire) and spreads all over. Then Vaayu absorbs Agni's attribute light and Agni's put out and wind starts blowing all around. Then sky absorbs the air's attribute "touch" and Vaayu becomes still and Aakaash (sky) becomes without any covering. At that time only sky is illuminated without any Roop (form), Ras, Sparsh (touch), and Gandh (smell). In the last, this sky's attribute "sound" is absorbed by Bhootaadi. Panchbhoot and Indriyaan are also absorbed in this Bhootaadi and then remains only Ahankaar. This Ahankar is Taamas, so it is absorbed by Saatwik Mahat Tattwa Buddhi.

As Prithvi and Mahat Tattwa are the two limits of inside of the Universe, in the same way they are the limits of the external world also. They also get absorbed in the same way. So the whole Universe - all the seven Dweep, seven seas, seven Lok, all are absorbed in water, water is absorbed in fire, fire is absorbed in air, air is absorbed in sky, and sky is absorbed in Bhootaadi Mahat Tattwa. Then all these are absorbed by Mool Prakriti.

Hey Muni, The equilibrium state of the three Gun (Sat Gun, Rajo Gun, and Tamo Gun) is called Prakriti or Pradhaan. This Pradhaan is the cause of the Universe. This Prakriti is both with attributes and without attributes, that is why its attributes - shown form are absorbed in its hidden form.

Apart from this Prakriti, there is a pure, A-Kshar, immortal and omnipresent which is also a part of Paramaatmaa. The same is in the whole Universe. When attained, Yogee doesn't come back to this world. The hidden Prakriti and Purush are also absorbed in Paramaatmaa. The same Paramaatmaa is described as Vishnu in Ved and Vedaant (Upanishad etc).

Vaidik (according to Ved) Karm is of two types - Pravritti (Karm Yog), and Nivritti (Saankhya or Gyaan Yog). Both types of Karm take one to the same Paramaatmaa. Rig Ved, Yajur Ved and Saam Ved worship that Paramaatmaa with Pravritti path; and whoever Yogee have adopted Nivritti path they also worship the same Yagya Purush. Whatever is said through these three sounds - Hriswa, Deergh and Plut, and whatever is not the subject of speech, all that is Vishnu only.

Whatever two Paraardh Time (Brahmaa's age) I have told you, that is Vishnu's one day only. When manifested Universe is absorbed in A-Vyakt (hidden) Prakriti, and Prakriti is absorbed in Purush, Vishnu has the night of the same "Time period" (two Paraardh time). In reality, that Paramaatmaa has neither day nor night, still it is said like this.

(3) Aatyantik Pralaya
When a person attains Moksh, it is called Aatyantik Pralaya

Naimittik or Avaantar Pralaya
Occurs in the end of two Paraardh (Brahmaa's age). Telling about Paraardh, Paraashar Jee says to Maitreya Jee that "Whatever number comes counting from one unit till 18 (digits) that is called Paraardh. So its double the number is called Praakrit Pralaya. At this time the whole Universe submerges in its cause A-Vyakt (which cannot be described, means God). When water goes beyond Saptarshi (Great Bear), this whole Tri-Lok becomes like huge ocean. The air emerged from Vishnu's mouth and His breath continue to blow up to 100 years. At that time Vishnu drinks all His Vaayu and sleeps with the help of His Maayaa power Yog Nidraa on His Shesh Naag bed. While this Pralaya occurs, sleeping of Bhagavaan Vishnu is called "Nimitt". Till He is awake, Universe's all activities go on and when He sleeps, the Universe also disappears in Him. When Brahmaa is awake all activities of the Universe again start in the same way as before.

Agni Puraan, 30 states that Pralaya is of for kinds -
(1) Nitya Pralaya - Who are born in the world they always die daily - this is called "Nitya" Pralaya.
(2) After 1,000 4-Yug period, Brahmaa's one day ends, at that time this Creation is absorbed - this is called "Braahm Laya" or "Naimittik" Pralaya [because it has a purpose - since the Creator sleeps so his Creation].
(3) When the 5 Bhoot (5 Elements) get absorbed in Prakriti - this is called "Praakrit" Pralaya. [because in this one, the Prakriti, Nature, also doesn't exist]
(4) When the Jeev is absorbed in Paramaatmaa after attaining Gyaan (knowledge) - it is called "Aatyantik" Pralaya - the final destruction, because after that the Jeev doesn't come back on Prithvi.