Picts seems to have formed themselves first as a confederation of peoples along the Roman limes, north of Roman Britain, in Late Antiquity.

The area where the Pictish confederation was formed fit an area known by Romans as Caledonia. It's not really clear what all these peoples were, but there was possibly a separation between British or British-related peoples, and others.Namely, looking at mentions of peoples by Tacitus, we have in the southern-western Lowlands peoples named Dumonii and Votadinii with a probable British character. It's far less clear for others as Venicones, Vacomagi or Taixali which lived in the eastern Lowlands. Maybe they were closely related to the first, maybe they weren't, the jury is still out. Roughly the main thesis are : all were Prittenic-speaking (northern Brythonic), they were a mix of British and pre-Celtic and/or Goidelic-speaking people.

As Agricola advanced in Scotland, he fought a "conspiration" (to be understood as a defensive league in the context) of local peoples called Calidones which gave the Roman name for the region.

The vague border was notriously unstable, unpractical and a waste of ressources that even Hadrian's Wall didn't really ended at first, as people from each side of the walls still maintained their old relations.

This context of perpetual unstability and conflict between natives and Romans probably helped reinforce alliances into more permanent groupings, namely Maetae and Calidones, whom rough border was roughly set at the Firth of Tay. After tentative raids, Septimus Severus defeated them, and enforced peace until the IVth century, the same way Romans did in continental Barbaricum : military defeat whenever possible, clientelisation, subsides if no other practical choice.

Caledonians (which included northern peoples) would be, then, no really different of other Barbarians groups that emerged and formed along the limes such as Franks or Goths, an assemblage of various peoples gathered by political interest, dynastic ties, and relations with Romans defining the mobilisation of elites (trough consumption and distribution of prestige goods)Picti, the painted ones, appeared as a way to distinguish submitted and "provincialized" Brittons (as in "Caledonians and other Picts"), from these unsubmitted, troublesome BrittonsAt the time of the Great Conspiracy, they were powerful enough to pose a challenge to the declining Roman Britain and overruned its northern part before their defeat, making a number on British peoples close to the Roman limes in the process,

With the collapse of Roman Britain, and the general militarisation of power, Picts didn't differed much from their neighbours, and due to the lack of written sources, most of what happened between the IVth and late VIth is speculative.What we know is that by 600, modern Scotland was partly Christianized (either trough Romano-British influence, either Irish, possibly both), and that as the process of establishment of petty-kingdoms continued northern kings took the lead of the Pictish ensemble.An Irish petty-dynasty ended up controlling both parts of Ulster and the south-western Pictish coast : the kingdom of the descendents of Reta, Dal Riata. It took a great regional importance, but was eventually part of the Pictish ensemble rather than just an "overseas" Ireland (even if links were never broken). Of course, more or less severe infighting for regional dominance tended to regularily happen.

Pictland seems to have been initially unaffected by what happened in England but Northumbrian hegemony build-up changed things : Pictish political centers might have shifted south, in relation with Northumbrians kings : Talorcan, nephew of Oswy, possibly have been put there by his uncle while minor kings probably entered in obedience of Northumbria as well when not replaced by Saxons.A rebellion took place in 671, led by a royal challenger from the North which was defeated by Northumbrians, which tentatively increased their presence, forcing Bridei (a relative of the Northumbrian king) out of Fife to rule from the North under their suzerainity, which backfired when the main king imposed aggressively his authority there before utterly defeating Northumbrians.It had the side-effect reasserting Fortiu's and northern dominance in Pictland, with ties re-established with Dal Riata : from this point, the main kings of the Picts were often half-Scottish even if ruling from the north of Pictland.

Picts, Brittons and Northumbrians regularily clash for the control of Alt Cult, and Picts among themselves for most of the period : eventually the hegemonic dynasty managed to impose itself to most petty kingdoms, including Dal Riata.

A mixed Gaelic-Pictish Pictland was emerging in the late VIIIth and early IXth century (an acculturation not just driven by sheer migration, but also intermixing, influence of Irish Christianity, etc. altough the reason remains largely debated and mysterious overall).Then Vikings happened, and they did a number on the emerging kingdom of Pictland. Not only ravaging the coasts, but beheading royal elites in 839 killing the king, his brother and a large part of their forces, but as well the king and those of Dal Riada. Followed by a short succession crisis, that kings of Dal Riata won and became both kings of Scots and Picts, which eventually fomed the Kingdom of Alba/Scotland.

By the time a certain feudality appeared in Scotland with the Davidian Revolution in the mid-XIIth, Picts had disappeared as a distinct people since more than three centuries.

What were Picts, eventually? First your typical Barbarian confederation born out of from border's relations with Romans (trade, raid, subsides, war, etc.), which grew into a series of petty-kingdoms slowly unifying and dominating their area, mixing with Gaelic people, and eventually building the foundations of what could have been known as Pictland today but ended up as Scotland.

Sources

- From Caledonia to Pictland, Scotland to 795, by James E. Fraser- From Pictland to Alba, 789-1070, by Alex Woolf