Neurofibrillary tangles are abnormal aggregates of protein that develop inside nerve cells, primarily associated with Alzheimer's disease. They are primarily composed of hyperphosphorylated tau protein, which normally helps stabilize microtubules in neurons. When tau proteins become excessively phosphorylated, they lose their ability to bind to microtubules, leading to the formation of tangles. These tangles disrupt normal cell function and contribute to the degeneration of neurons. Their presence is considered a key pathological feature of Alzheimer's disease and is used in research to understand the progression of neurodegenerative disorders.