The Lord Commander of the Kingsguard is the head of the Kingsguard, appointed by the king himself. In the case of a female sovereign (and thus a Queensguard rather than a Kingsguard), the title is Lord Commander of the Queensguard.
The king usually appoints a new Lord Commander from one of the current serving members of the Kingsguard, though there may have been exceptional cases in which a knight was instantly appointed to the Kingsguard as well as the position of Lord Commander.
The Lord Commander wears the same uniform armor as the regular members of the Kingsguard, with no difference in appearance or regalia. Given that there are only seven Kingsguard members at any one time, including the Lord Commander himself, it is usually an easy matter to keep track of their identities at court and on the battlefield.
The Lord Commander of the Kingsguard also holds a seat on the king's Small Council by virtue of his office. His role on the council is to give military advice on land-based warfare (separate from the Master of Ships, who handles naval affairs). During the reign of King Robert Baratheon , the position was stripped of it's small council seat. The seat however, was restored to the small council under King Bran Stark.
Unlike several other offices on the Small Council the Lord Commander of the Kingsguard cannot be lawfully dismissed, or at least not totally dismissed from the Kingsguard, because their vows are taken for life. Theoretically a king might appoint one of the other six Kingsguard members as the new Lord Commander to replace him, but it is unknown if this has ever happened.
While reading through the White Book, Jaime notes that most of the Kingsguard served adequately, but few people remember their names generations later. The only ones people remember centuries later are those who were the best (great heroes such as Aemon the Dragonknight), and those who were the worst (either traitors to the crown or those who were derelict in their duty, broke their vows and fathered children). Jaime also notes that on rare occasion, some of the past Lord Commanders were a bit of both, particularly Ser Criston Cole, perhaps the most infamous Kingsguard member who ever lived until Jaime himself. He started out as a favorite of Princess Rhaenyra Targaryen, serving as Lord Commander of her father Viserys I's Kingsguard, but eventually had a falling out, and he joined her enemies at court. Conflicting stories from the banal to the lurid and extreme exist about exactly what happened between them, ranging from that Criston was too noble to break his vows of chastity, to that he eagerly offered to flee with her to the Free Cities as a common sellsword, but Rhaenyra thought that only a Kingsguard was worthy of her. Others say that it was actually Rhaenyra who threw herself at Criston, attempting to sexually seduce him and desperate to lose her virginity to him rather than in her arranged marriage to her homosexual cousin Laenor Velaryon - after which he grew disgusted and spurned her, and she took Harwin Strong as a new lover. Whatever the cause, Criston joined the rival faction at court championing her half-brother Aegon II, and upon the death of her father, was an active leader in the conspiracy, personally slitting the throat of the Master of Coin when he protested crowning Aegon II instead of Rhaenyra. Cole then personally placed the crown on Aegon II's head at his coronation, and was forever after remembered as "the Kingmaker". Some say that Cole supported Aegon II in the civil war that followed out of petty revenge against Rhaenyra, while some say he did it out of respect for Andal laws which place a son before a daughter in inheritance (despite the fact that her father had repeatedly and publicly made the high lords of the realm swear that Rhaenyra would rule after him). Still others say that Criston had no great love of Aegon II, yet was not driven out of petty hatred for Rhaenyra, but rather because he had honestly concluded that Rhaenyra was too unstable to rule (particularly due to her new husband Daemon Targaryen), and that the Small Council could more easily control Aegon II as a puppet ruler.