Great job. We've covered a lot in this chapter.
Just as we did with web3, the best way to learn or catch up on Solidity is to play with it inside of an isolated environment. I've created a file for you called solidity-playground.sol. You can find it here: https://github.com/PacktPublishing/Truffle-Quick-Start-Guide/blob/master/chapter2/solidity-playground.sol. This contains the same SimpleTransfer contract. It's for you to play with, add/alter functions, and so on—it's all yours. You can use an online Solidity IDE called Remix (https://remix.ethereum.org) to run your contract.
See you in Chapter 3, Choosing an Ethereum Client for Your Dapp, where you will learn how to interact with various Ethereum clients and choose the correct one for your Dapp.