What are the basic features of CIDR and what are its advantages and disadvantages compared to IPv6?

CIDR stands for Classless Interdomain Routing and was introduced in 1993. The goal of introducing CIDR was to reduce the number of routing table entries by enabling route summarization and supernetting and make better use of the available IPv4 address space.

The notation format for an address is different with CIDR. What we know as the subnet mask is noted in number of network bits appended to the IPv4 address.

Old notation: 192.168.0.0 mask 255.255.255.0
In CIDR notation: 192.168.0.0/24

You can find detailed information in RFC 1518 and 1519. The address specification of IPv6 is different from the IPv4 specification. An IPv6 address has a prefix, which is usually composed of a provider-allocated public prefix, a 16-bit subnet prefix for internal subnets and a 64 bit interface identifier. The prefix length is appended with a slash, specifying the number of prefix bits. It looks just like CIDR.

Check RFC 3513 for the IPv6 address specification.