A distributed ledger is a consensus of replicated, shared, and synchronized digital data geographically spread across multiple sites, countries, and/or institutions.
Sometimes called private blockchains, allow for distributed identical copies of a ledger, but only to a limited amount of trusted participants only. As the network may have an owner(s), this methodology is better suited for applications requiring simplicty, speed, and greater transparency.
A blockchain is a type of distributed ledger, comprised of unchangable, digitally recorded data in packages called blocks.
Open Source Standards
The future of mainstream blockchain technology will likely not result in one blockchain to rule them all. There will likely be many implementations of distributed ledger technology and it is to everyones benefit that they all be interoperable with eachother. The result of this is emerging open source, neutral protocols and standards for various implementations.
Also called unpermissioned ledgers, allow anyone to contribute data to the ledger with all participants possessing an identical copy of the ledger. Since there is no single owner of the ledger, this methodology is more suitable for censorship resistant applications (e.g. Bitcoin).
Smart contracts are contracts whose terms are recorded in a computer language instead of legal language. Smart contracts can be automatically executed by a computing system, such as a suitable distributed ledger system.