What are the different types of collections and what are maps in Salesforce Apex?

The collection is the type variables which can store multiple numbers of records. It can increase and decrease dynamically.

There are 3 types of collection
List Collection
Set Collection
Map Collection

A map is a collection of key-value pairs where each unique key maps to a single value. Keys and values can be any data type—primitive types, collections, sObjects, user-defined types, and built-in Apex types.

Example:

public static void beforeUpdateHandler(Map<Id, Account> oldMap, Map<Id, Account> newMap) {
Account oldAcc;
Account newAcc;
for (Id accId : oldMap.keySet()) {
oldAcc = oldMap.get(accId);
newAcc = newMap.get(accId);
if (oldAcc.MyCustomField__c != newAcc.MyCustomField__c) {
System.debug('MyCustomField__c has changed!');
}
}
}

Thanks