1. Introduction

Welcome to this introductory tutorial on decentralized applications (dApps). The primary goal of this tutorial is to equip you with a basic understanding of dApps, including their characteristics, types, and principles.

By the end of this tutorial, you should be able to:
- Understand what dApps are and their characteristics
- Identify the different types of dApps
- Understand the underlying principles of dApps
- Start developing your own simple dApp

Prerequisites:
- Basic understanding of blockchain technology
- Basic programming knowledge, preferably in JavaScript

2. Step-by-Step Guide

2.1 What are dApps?

Decentralized applications (dApps) are applications that run on a P2P network of computers rather than a single computer. They are similar to traditional web applications. The backend code of dApps run on a decentralized peer-to-peer network (blockchain), while the frontend code can be written in any language that can make calls to its backend.

2.2 Characteristics of dApps

• Open Source: The application's source code is available to all, and changes are decided by consensus.
• Decentralized: Data and records are stored on a public and decentralized blockchain to avoid pitfalls of centralization.
• Incentive: Validators of the blockchain are incentivized by rewarding them accordingly with cryptographic tokens.
• Protocol/Algorithm: The application community must agree on a cryptographic algorithm to show proof of value.

2.3 Types of dApps

There are three types of dApps:
1. Type I dApps: These have their own blockchain (like Bitcoin).
2. Type II dApps: These are protocols and have tokens that are necessary for their function. They use the blockchain of a Type I dApp.
3. Type III dApps: These are protocols/apps that use the protocol of a Type II dApp.

3. Code Examples

3.1 A Simple Smart Contract in Solidity (Ethereum dApp)

Solidity is a high-level language for implementing smart contracts on platforms like Ethereum.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract SimpleStorage {
    uint data;

    function set(uint x) public {
        data = x;
    }

    function get() public view returns (uint) {
        return data;
    }
}

Explanation:
- pragma solidity ^0.8.0;: This code indicates the version of Solidity we're using.
- contract SimpleStorage {}: This creates a contract named SimpleStorage.
- Inside the contract, we have a state variable data of type uint (unsigned integer).
- set(uint x): This function allows us to set the value of data.
- get(): This function allows us to return the current value of data.

4. Summary

In this tutorial, we covered the basics of dApps including their characteristics, types, and principles. We also saw how to write a simple smart contract on Ethereum. The next step would be to dive deeper into Ethereum, learn more about Solidity, and start building your own dApps.

5. Practice Exercises

1. Exercise 1: Write a simple Smart Contract to store and retrieve a string value.
2. Exercise 2: Modify the above smart contract to include a function that can modify the string value stored.

Solutions:
1. Solution to Exercise 1:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract SimpleStorage {
    string data;

    function set(string memory x) public {
        data = x;
    }

    function get() public view returns (string memory) {
        return data;
    }
}

1. Solution to Exercise 2: The solution will be the same as Exercise 1, as the set function can modify the stored string.

Tips for Further Practice: Start building more complex contracts and experiment with Ethereum's features. You can also explore other platforms for creating dApps, such as EOS or Tron.