What little I know is that they were raiders who came from somewhere unknown, turned up, had some pillaging fun, and then just... went away. Have we found out more? My understanding is that the civilizations attacked were also clueless.
Several of the groups of "Sea Peoples" (which I do not consider a useful term) appear as early as the Amarna period, well over 100 years before the raids in the Ramesside period. They did not simply appear out of nowhere, and we should be careful not to compress and simplify over a century of events and cultural-ethnic interaction. For example, the Karkiša and Lukka fought on behalf of the Hittites in the Battle of Kadesh.
Moreover, the "Sea Peoples" were intent on settling coastal areas, not simply raiding. We now know, for example, that they brought their livestock with them, and the Egyptian Tale of Wenamun mentions that the Tjekker have settled in Dor (in modern Israel).
The groups lumped into the category of "Sea Peoples" include the Denyen, Eqwesh, Karkiša, Lukka, Peleset, Shekelesh, Sherden, Teresh, Tjekker, and Weshesh. Some of these have been tentatively identified with locations in the Mediterranean, including the Shekelesh (Sicily), Sherden (Sardinia), and Karkiša (Caria). Since these identifications remain speculative, however, I will focus on the origins of the three groups that have been identified with some certainty, the Peleset, Lukka, and Eqwesh.
The Philistines (Egyptian Prst/Plst) almost certainly came from the Aegean. I discussed the Philistines in Tell me about the Philistines vs others in the Levant 1200 to 800 BCE.
Turning to the Lukka, the "Lukka lands" are probably to be identified with classical Lycia in southwestern Anatolia. In the Tawagalawa letter of the 13th century BCE, a letter from the Hittite king to a (the?) Mycenaean king, the Hittite king writes about the Lukka people of western Anatolia.
Then when the people of Lukka appealed to Tawagalawa, he went to those lands. They likewise appealed to me, so that I came down to those lands. When I arrived in the town of Sallapa, he (Piyamaradu) sent a man to meet me, with the message: “Take me into your service! Send the Crown Prince to bring me to your Majesty!” I sent the Crown Prince to him: “Go set him on the chariot with you in order to bring him here.” But that one squelched the Crown Prince and refused. But isn’t the Crown Prince the social equal of the King? He held his hand, but in response he refused. He belittled him before the lands. And on top of this he said: “Bestow kingship on me here on the spot! If not, I will not come.”
In a letter from the Hittite king to King Ammurapi of Ugarit, the Hittite king wrote about the delayed shipment of copper to Ahhiyawa-men (Mycenaeans) stationed in Lukka.
Now i have been told that the Ahhiyawan is tarrying in the land of Lukka, but that there are no copper ingots for him. in this matter don’t tell me that there is no appropriate action. Give ships to Satalli, so that he may take the ingots to the Ahhiyawans.
The Eqwesh have been identified with the Ahhiya/Ahhiyawa of Hittite texts, which has in turn been identified with the Homeric Achaeans and therefore Mycenaean Greece. The Ahhiyawa appear in quite a few Hittite texts:
Fragmentary sequence of oracular inquiries that ask about military actions and an “enemy man [i.e. leader/ruler] of Ahhiya” in addition to rituals for various deities and other affairs.
The king reminds Madduwatta that he saved him from Attarissiya, the man of Ahhiya.
• Madduwatta was given the region of Mt. Zippasla to rule and swore an oath with Hatti.
• Madduwatta attempted to seize territory from Kupanta-Kurunta of Arzawa but was defeated. The Hittites sent reinforcements, and Kupanta-Kurunta fled.
• Attarissiya attacked Madduwatta once again, prompting the Hittites to send Kisnapili with troops and chariots to support Madduwatta. Attarissiya returned to Ahhiya.
• Madduwatta betrayed the Hittites by not attacking Dalauwa and revealed their position at Hinduwa. He forced the citizenry of Dalauwa to swear allegiance to him.
• Madduwatta plotted with Kupanta-Kurunta and possibly married his daughter. He attacked various Hittite towns and encouraged them to rebel against the king.
• Arnuwanda sent Mulliyara to treat with Madduwatta, whose reply is lost/broken.
• Madduwatta seems to have raided Alashiya/Cyprus, supposedly under Hittite control.
The annals describe Mursili’s campaigns in Arzawa. There is a mention of a “king of Ahhiyawa” (LUGAL KUR Ahhiyawa) and a messenger being sent with a ship.
There is a reference to Uhha-Ziti, a king of Arzawa, who supported (lit. “stood behind”) the king of Ahhiyawa in conjunction with the land of Millawanda. Hittite troops captured captives, cattle, and sheep and brought them back to Hattusa. Manapa-Tarhunta of the Seha River Land initially followed Uhha-Ziti in his alliance but begged Mursili II for mercy upon his approach, which he granted only after the mother of Manapa-Tarhunta personally begged him for clemency. Uhha-Ziti’s son Piyama-Kurunta was ultimately defeated, but Uhha-Ziti evaded capture and escaped to islands in the Mediterranean. Mursili II succeeded in eliminating Arzawa as a serious threat.
Series of oracular inquiries. One inquiry asks about the god of Ahhiyawa, Lazpa (Lesbos), and the king. When it is asked whether a rite should be performed for three days for the gods of Ahhiyawa and Lesbos as for the Hittite deities, the answer is yes.
A reference is made to “in the land of Ahhiyawa” and the sea.
Extremely fragmentary. The letter includes the phrase “they brought from Ahhiyauwa.”
The Hittite general Kassu brought troops to the Seha River Land before leaving to attack Wilusa. There is a reference to Piyamaradu, who installed Atpa as king and attacked Lesbos, causing the dyers to switch allegiance to Atpa.
This extremely interesting letter is from a “great king” (LUGAL.GAL) of Ahhiyawa. He refers to islands, seemingly property transferred through some union with Assuwa. The writer notes that Tuthaliya I/II, the “father of the father of your father” defeated the king of Assuwa but that the islands were indeed the property of the king of Ahhiyawa.
The king notes that Piyamaradu demanded to speak with the crown prince but then declined to meet with him. The Hittite king arrived in Iyalanda, where he repelled the people who attacked him. Afterwards the king wrote to Piyamaradu in Millawanda requesting his presence, but missive was ignored. The Ahhiyawa king wrote to Atpa in Millawanda on his behalf demanding that Piyamaradu be handed over to the Hittite king, but Piyamaradu, the father-in-law of Atpa, was able to escape in a ship to Ahhiyawa-occupied territory. The Hittite king requests that Piyamaradu be handed over to Hittite authorities or forcibly kept in Ahhiyawa with no further hostile actions against Hatti. Tawagalawa, the brother of the king of Ahhiyawa, has attracted interest primarily due to his name’s resemblance to Eteokles (Mycenaean E-to-wo-ke-le-we).
The letter references a gift for the king of Ahhiyawa. The writer has taken a silver rhyton and a rhyton of inlaid gold from the gifts intended for Egypt and instead sent them to the king of Ahhiyawa.
Fragmentary but mentions Piyamaradu and the king of Ahhiyawa.
Votive prayer of (probably) Puduhepa (CTH 590, KUB 56.15)
Boundary list (?) of a Hittite king (CTH 214.16, KUB 31.29)
The land of Ahhiyawa is mentioned in a list along with Tarhuntassa and Mira.
The letters reveals that Urhi-Teshub made peace overtures to Ahhiyawa, but the king of Ahhiyawa and at least one other region refused assistance.
The Seha River Land, quiescent since the reign of Mursili II, broke into rebellion under Tarhunta-radu and was backed by the king of Ahhiyauwa.
The king requests that Walmu be turned over to him so that he can be reinstated as the king of Wilusa. Piyamaradu is also mentioned.
And the kings who are my equals...the king of Egypt, the king of the land of Babylonia, the king of the land of Assur, and the king of the land of Ahhiyauwa...
The letters, found in the “house of Urtenu,” are from Suppiluliuma II and the official Penti-Sarruma. Ammurapi is criticized for not shipping PAD.MES (metal ingots?) the Hiyawa men in the land of Lukka.
The letter references “my brother, the king of Ahhiyawa.”
More can be written, but you might like to see the FAQ's section titled "Sea peoples" (in the app, the # anchor doesn't work, so you have to find it a little over halfway down the page).
This is not to discourage discussion. Further questions, data, and debate are welcome.