The cantigas de amigo, or "songs of a lover," are a genre of lyric poetry written in Galaic-Portuguese (a language ancestral to modern Portuguese and Galician) during the 12th and 13th centuries in the northwestern Iberian, reaching its zenith during the reign of Alfonso X of Castile. Their structure and content contrasts with contemporary poetry from other regions, as well as with other Galaic-Portuguese lyric, making it probable that they reflect an indigenous Iberian tradition of love poetry. Indeed, the only obvious parallel is with the Mozarabic kharja tradition, also unique to the Iberian.
In a cantiga de amigo, the poet (almost always male, in extant examples, though it's not clear how well this reflects the popular tradition) takes on the voice of a young woman, generally one of common background. This narrator is generally presented confiding in someone close to her, often her mother, about her love. (The word amigo, literally "friend," is commonly used to refer to the lover, but there's very rarely any ambiguity about whether romance or mere friendship is being discussed. It's therefore probably better to interpret the phrase cantigas de amigo as meaning "songs of a lover," rather than the more popular translation as "songs of a friend.")
Using this device, the poets captured glimpses of many moments in romance. Some cantigas speak of unrequited love, some of lovers who have gone to sea and may never return, some of nervously hoping to see a lover again who may not be faithful after all. Others are more joyous; in some, the lovers are planning to meet, or the woman simply sings of how joyful it is to be in love. Many hint specifically at erotic love, with plans for secret meetings or lovers bathing together in the ocean; a few go beyond hints. The sentiments are poignant, but usually quite simple; each poem deals only with one precise theme.
The content of these poems differs from that of the troubadour-inspired lyrics the same poets were often writing when they favored a male voice. The male-voice poems are frequently about longing for a noble lady whose love is unobtainable, a common theme in other Romance-language poetry of the era, while the cantigas de amigo are generally about an uncertain but more promising romance, and often present a more raw quality to the emotional expression.
The structure of the cantigas de amigo is also unique. Unlike other Romance-language poetry of the same period (even in the same region), most of the cantigas de amigo are highly repetitive; in general, there's a high degree of parallelism from one stanza to another, often concluding each one with a shared refrain. This tends to create the effect of a narrator so filled with emotion that she can't quite bring herself to find the right words, but she continues to try. But despite these similar structural devices, there isn't a single, fixed cantiga de amigo form. A two-stanza cantiga is short, but not unheard-of; four stanzas is more typical, and some are substantially longer. There is also no set form that the poems take within those stanzas; the stanzas themselves may be as short as three lines, but most are longer, and the longest I'm familiar with are eight lines long. The lines themselves are typically brief, in the manner of a lot of folk poetry; seven-syllable lines are common (though not ubiquitous), and rhyme schemes vary.
The fact that widespread features of European court poetry (which was, at the time, strongly influenced by the troubadour tradition) made it into all parts of the Iberian, yet the cantigas de amigo failed to absorb those details and stayed confined to one relatively small region, suggests that they are an indigenous popular form, of which many artful examples remain to us precisely because it was adopted by high-status poets. While we have no way to discern whether the lower-class women whose voices are often adopted in the cantigas de amigo were ever part of that tradition, these poets certainly give us a glimpse of the distinctive style of verse that was often composed and sung in the northwestern Iberian. It also shows us that the local form those courtly poets chose to present to a wider audience was one that specifically celebrated the many emotions that love inspires.