How is hashing used in blockchains?

Hashing is used to link blocks, create public/ private key pairs, and in proof of work blockchains, solve complex algorithms.

What is the difference between second pre-image and collision resistance?

With second pre-image resistance, an input is known and the attacker is able to find a second input that hashes to the same value. With Collision resistance, both inputs are unknown, and the attacker is able to find both inputs that hash to the same value. Collision resistance implies pre-image resistance.

A hash function is any function that can map arbitrary-size data to fixed-size values. Values returned by a hash function are called hash values, hash codes, digests, or simply hashes.

How are hashes used everyday?

Many people unknowingly use hashes when logging into an account. When you create an account on an application, the provider more than likely does not save your password; they save a hash of your password. When you sign in to your account, the app hashes the password you enter and compares this hash to the hash it has saved on file.