Ingrown toenails (known in medicine as Onychocryptosis) are an extremely common (and often very painful) problem for people of all ages and levels of activity. It occurs when a portion of the nail plate digs in or punctures the surrounding skin. Left untreated, ingrown toenails will often become even more painful and can become infected.
Pain is, as you would guess, the almost universal feature of an ingrown toenail. The damage inflicted to the adjacent skin can also lead swelling, redness and, very often, localised infections. As swelling around the offending piece of nail increases, the nail is enveloped even further and therefore digs in more, leading to a viscous cycle and potentially severe versions of the symptoms described.