The original shield was triangular in shape, and was given to King T'Chalka of Wakanda as a pledge of Captain America's good faith in helping to keep Wakanda unspoiled and undiscovered during World War II. That shield is still a Wakandan national treasure.
The second triangles shield was 'retired' in favor of the round shields, and kept in Army storage with Steve Roger's other effects, eventually to be returned to him. It was destroyed by Mr. Hyde during the seige at the Avengers' Mansion by the Masters of Evil, led by Baron Zemo II.
The round shield which was to become the mainstay of Captain America's career was developed by Dr. Myron MacLain, who had been researching the creation of an indestructable alloy for use in making American armored fighting vehicles. The shield was made from a base steel alloy mixture, vibranium, and a "mystery metal" (an alloy which has yet to be successfully reproduced) which crept in while he was asleep. The shield is harder and more durable than adamantium - adamantium itself was discovered by Dr. MacLain when he unsuccessfully tried to replicate the amalgamation. Several beings of incredible physical strength have tried to damage it (including an enraged Hulk), but to no effect. It was destroyed, and then re-created by the Beyonder in the Secret Wars, and the recreation has a submolecular fault which would ultimately allow it to be smashed to small pieces. These pieces were reassembled, and used to absorb the supra-vibratory energy of Klaw, resulting in the the re-fusing of the shield (and Klaw's defeat).
Physical Statistics: The shield is a concave disc, 30" in diameter, weighing twelve pounds.
Shortly after Captain America was reawakened to modern times, he loaned the indestructable shield to Tony Stark for analysis, and for a time used several steel substitutes. The first of these had microcircuitry inside, and CA controlled its flight with powerful electromagnets. However, the circuitry made the substitute shields off-balance, and CA used solid steel replicae. One of these was destroyed by the Living Laser in an early issue of the Avengers. Given how relatively fragile these shields were, CA stocked the Avengers vehicles with several spares.
Captain America II and III each used steel replicae of the indestructable shield. The shield used by CA III was held with him while he was in suspended animation, and shattered in combat with Captain America I
When the government took the Captain America identity and shield from Steve Rogers, he adopted the identity of the Captain, and used an adamantium shield provided by Tony Stark. After the conflict between Iron Man and Captain America (during the Armor Wars), Steve got rid of this shield.
After Steve Rogers (as the Captain) got rid of the Stark shield, he was given a vibranium alloy shield by the Black Panther. This shield would presumably be carried by John Walker as the USAgent, when he adopted the costume and shield after his stint as Captain America.
During Steve Roger's sojourn as a "Man Without A Country", Sharon Carter would supply him with a photonic energy shield.
After the indestructable shield shattered due to an instability in the adamantium-vibranium alloy, Captain America would try three shields before the indestructable shield was restored. These included:
An adamantium shield which had too much "bounce", and did not pass testing in the Avengers' Mansion.
A new triangular shield.
A new photonic energy shield with a flexible containment matrix, allowing CA to shape it into different forms as desired, ie a staff, a shield, a sword, etc.
