What are Local and Global ids with respect to lightning component in salesforce?

The component has two types of IDs: a local ID and a global ID:
Local ID: It is an ID that is only scoped to the component. It is often unique but it’s not required to be unique. Local ID is created by using the aura:id attribute. E.g.: To find the local ID for a component in JavaScript, use cmp.getLocalId()
Global IDs: Every component has a unique global Id, which is the generated run=me-­‐unique ID of the component instance.