Blockchain Ledger is a data structure similar to linked list, every block is a container linked to each other cryptographically.
Blockchain Technology is a decentralized computation and distributed ledger platform to immutably store transactions in a verifiable manner efficiently, through rational decision making process among multiple parties in an open and public system.

Decentralized Application (DApp) is a computer application that is deployed and gets executed on a Distributed system usually Blockchain or Distributed Ledger Technology. DApp is a set of Smart Contracts interfaced with a UI.
Some features of DApp as defined by the Ethereum

• Decentralized
• Deterministic
• Turing complete
• Isolated

• Zero downtime: Once the smart contract of the Distributed App is deployed on the blockchain, No malicious actors can launch a Denial of Service because the blockchain network as a whole will always be able to serve clients with the smart contracts.
• Privacy: Blockchain accounts and transactions are pseudo-anonymous there is no need to provide real-world identity to deploy or interact with a dapp.
• Resistance to censorship: The application cannot be controlled by a single entity on the network or stop any users from submitting transactions, deploying DApps, or reading data from the blockchain.
• Data Integrity: Just like the ata stored on the blockchain is immutable and indisputable, this property holds good for the smart contracts deployed in Blockchain.
• Verifiable Behavior: As smart contracts can be analyzed and are guaranteed to execute in predictable ways, the same holds good for DApp also

• Maintenance can be harder because of immutability of smart contracts. So any security bugs or updates cannot be made once the contracts are deployed.
• Performance overhead due to the choice of consensus algorithm and storage.
• Network congestion is a major implication at least in the current model. Even if a DApp is using too many computational resources, the entire network gets congested. In Ethereum, the network is only able to process about 10-15 transactions per second; if transactions are faster than this, the pool of unconfirmed transactions and transaction fee shall go high.
• User experience atleast at present is harder to provide user-friendly experiences. The average end user might find it too difficult to set up necessary tool stack necessary to interact with the blockchain in a truly secure fashion

pragma solidity ^0.8.0;

contract SimpleDApp {
    address payable owner;
    mapping(address => uint) balances;

    constructor() public {
        owner = msg.sender;
    }

    function deposit() public payable {
        require(msg.value > 0, "Deposit amount must be greater than 0");
        balances[msg.sender] += msg.value;
    }

    function withdraw(uint amount) public {
        require(amount > 0, "Withdraw amount must be greater than 0");
        require(amount <= balances[msg.sender], "Insufficient funds");
        msg.sender.transfer(amount);
        balances[msg.sender] -= amount;
    }

    function getBalance() public view returns (uint) {
        return balances[msg.sender];
    }
}

• CryptoKitties: A blockchain-based game that allows users to collect and breed virtual cats.
• Augur: A decentralized prediction market platform that allows users to create and trade predictions on the outcome of events.
• OpenSea: A marketplace for buying and selling digital collectibles such as in-game items and virtual real estate.
• Uniswap: A decentralized exchange (DEX) that allows users to trade cryptocurrencies without the need for a centralized intermediary.
• MakerDAO: A decentralized lending platform that uses a stablecoin called DAI to provide loans in a decentralized manner.
• Ethlance: A decentralized freelance marketplace that allows users to find and hire freelance talent in a trustless manner
• Compound: A decentralized finance (DeFi) platform that allows users to lend and borrow cryptocurrencies.
• Steemit: A blockchain-based social media platform that rewards users for creating and curating content.

• EOS: EOS is a blockchain platform that is designed for high-performance decentralized applications. Some examples of DApps built on EOS include:
  • PRA CandyBox: A blockchain-based game that allows users to earn rewards by playing mini-games.
  • Everipedia: A decentralized version of Wikipedia that allows users to earn rewards for contributing content.
• TRON: TRON is a blockchain platform that aims to build a decentralized internet. Some examples of DApps built on TRON include:
  • TRONbet: A decentralized gambling platform that allows users to place bets on various games.
  • BitTorrent: A decentralized file-sharing platform that allows users to share files without the need for a centralized intermediary.
• IOTA: IOTA is a blockchain platform that is designed for the Internet of Things (IoT) applications. Some examples of DApps built on IOTA include:
  • IOTASeed: A decentralized seed generation service that allows users to generate seeds for their IOTA wallets in a secure and private manner.
  • IOTA Identity: A decentralized identity management system that allows users to create and manage their digital identities in a secure and private manner.
• Cosmos: Cosmos is a blockchain ecosystem that allows for interoperability between different blockchains. Some examples of DApps built on Cosmos include:
  • Terra Station: A decentralized exchange that allows users to trade different assets across different blockchains.
  • ChainGuard: A decentralized security platform that allows users to secure their assets on different blockchains.