The "a" in Cat 6a stands for "Augmented." In comparison to the regular Cat 6 cables, 6a cables support twice the maximum bandwidth and are capable of maintaining higher transmission speeds over longer cable lengths. Cat 6a cables are always shielded, and their sheathing — which is thick enough to eliminate crosstalk completely — makes for a much denser, less flexible cable than Cat 6.
Cat7 cables utilize the newest widely-available Ethernet technology and support higher bandwidths and significantly faster transmission speeds than Cat 6 cables. They're proportionally more expensive than other Ethernet cables, though their performance reflects their premium price tag. Cat 7 cables are capable of reaching up to 100 Gbps at a range of 15 meters, making them an excellent choice for connecting modems or routers directly to your devices. Cat 7 cables are always shielded, too, and use a modified GigaGate45 connector, which is backwards compatible with regular Ethernet ports.