A lot of people who join the crypto space talk about DApps and Smart Contracts and they use them interchangingly. DApps and Smart Contacts are closely related to each other but they are not one and the same thing. DApp is just an App but it is decentralized. Smart Contracts are small programs that contain the business logic on the blockchain that determine how the value gets exchanged autonomously without the involvement of a trusted intermediary.
Smart Contracts
Smart contracts are programs executed in a decentralized fashion on the blockchain, such as EVM(Ethereum Virtual Machine). They can hold value, like money and decide what to do with it. They can hold it until a certain condition is met and release it otherwise, so you don't need escrow services, etc. You can put the small pieces of code that you need on the blockchain and build the entire architecture around the Smart Contract as a DApp.
DApp
DApp is an architecture that does not need to be on the blockchain entirely. It provides the UI, holds the database, has a backend and it has one or more Smart Contracts integrated into it. You can also have several DApps imtegrated with one Smart Contract. For instance, one Smart contracts deals with your cryptocurrency, the other one takes care of the ownership or subscription of certain services within the DApp. There are countless ways you can use Smart Contracts within your DApp.
Currently, there are no browsers that support the Smart Contracts natively. So, you always need some sort of Plugins, such as 'Metamask'. There are tools like Web3.js that allow you to link your website or WebApp with the Smart Contracts. I will cover them in detail in the later parts. I hope you understand the difference between the DApps and the Smart Contracts.
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Introduction To 'Solidity'(Learn 'Solidity' - Part 1)
Ethereum & Smart Contracts(Learn 'Solidity' - Part 2)
Tips To Learn 'Solidity' On Your Own(Learn 'Solidity - Part 3)
Basic Smart Contract(Learn 'Solidity' - Part 4)
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Variables In 'Solidity'(Learn 'Solidity' - Part 6)