How to access Ethereum?

Because a blockchain network is a separate network, different and distinct from traditional Internet connections, specific software is necessary to access the network and show the data being recorded on the blockchain. This is possible through an Ethereum client, which is standalone software, often command-line interface only, and is the tool of choice for many developers. Additionally, the Ethereum community has developed a number of resources that allow connections between the traditional Internet and the Ethereum network. The backbone of these efforts has been MetaMask, a trailblazing browser plugin and mobile app that provides users with a custodial (user-controlled and owned) Ethereum wallet and access to dapps, or decentralized apps, that allow you to interact with the Ethereum blockchain.

What is a block explorer?

When you want to go deeper and see details of individual transactions, or take a birds' eye view of the Ethereum network, a block explorer is a website that provides an interface to navigate and examine the information contained on the network. Prominent examples include Etherscan and Ethplorer.

Ethereum mainnet, testnets, sidechains and the rest

As you venture forth into the decentralized web, or Web 3.0 as it's often called, you will learn that Ethereum is, in fact, not just one network. The Ethereum blockchain and the EVM live and operate on the Ethereum mainnet, and there exist a number of testnets for Ethereum that are exactly what they sound like, sandboxed versions of the mainnet where ETH has no real value except to test out applications.
That's just the beginning, though; there are many Ethereum-compatible sidechains that have been developed, offering users the option to carry out transactions on a separate blockchain, in that chain's native currency, in order to avoid the sometimes-costly EVM and Ethereum mainnet transaction fees. Users often end up with tokens and NFTs on these sidechains that they can bring back onto Ethereum mainnet; the NFTs can be kept, displayed, sold on marketplaces; the tokens can be swapped for others, redeemed for ETH, staked, borrowed, lent, used in other dapps, transferred to other sidechains and used in videogames, or videogame-investment engine hybrids, or, well... the future is still being written.