The EOSIO platform provides a number of components and libraries which are used to operate blockchain nodes, collect blockchain data, to interact with these nodes, and to build smart contracts. The main component is nodeos (node + EOSIO = nodeos). This is the core EOSIO node daemon that can be configured with plugins to run a node. Example uses are block production, dedicated API endpoints, and local development. Cleos (CLI + EOSIO = cleos) is a command line interface which interacts with nodeos, allowing you to send commands and actions to a blockchain. Cleos also interacts with keosd (key + EOSIO = keosd), a local component that securely stores EOSIO keys.
To build smart contracts you require EOSIO.CDT, the Block.one developed toolchain and libraries, which generate the WebAssembly binary instructions or bytecode into wasm files. The generated wasm files are the smart contracts which can be deployed to EOSIO blockchains.
The basic relationship between these components is illustrated in the following diagram:
EOSIO also provides a frontend library for javascript development called EOSJS along with Swift and Java SDKs for native mobile applications development.
Nodeos
Nodeos is the core EOSIO node daemon. Nodeos handles the blockchain data persistence layer, peer-to-peer networking, and contract code scheduling. For development environments, nodeos enables you to set up a single node blockchain network. Nodeos offers a wide range of features through plugins which can be enabled or disabled at start time via the command line parameters or configuration files.
You can read detailed documentation about nodeos here.
Cleos
cleos is a command line tool that interfaces with the REST APIs exposed by nodeos. You can also use cleos to deploy and test EOSIO smart contracts.
You can read detailed documentation about cleos here.
Keosd
keosd is a key manager daemon for storing private keys and signing digital messages. keosd provides a secure key storage medium for keys to be encrypted in the associated wallet file. The keosd daemon also defines a secure enclave for signing transaction created by cleos or a third party library.
keosd can be accessed using the wallet API, but it is important to note that the intended usage is for local light client applications. keosd is not for cross network access by web applications trying to access users' wallets.
You can read detailed documentation about keosd here.
EOSIO.CDT
EOSIO.CDT is a toolchain for WebAssembly (Wasm) and a set of tools to facilitate contract writing for the EOSIO platform. In addition to being a general-purpose WebAssembly toolchain, EOSIO-specific optimizations are available to support building EOSIO smart contracts. This new toolchain is built around Clang 7, which means that EOSIO.CDT has most of the current optimizations and analyses from LLVM.
EOSJS
A Javascript API SDK for integration with EOSIO-based blockchains using the EOSIO RPC API.