The lookup field helps in establishing a relationship between different modules (linking two different modules: e.g, Accounts with Contacts). For example, say some of your contacts refer more leads for your business. You want to know which contact referred a lead. So you have a field called "Referred By" in your Leads module, which lets you browse for and choose the correct contact. This association is established by creating a Lookup field. Note that lookup fields cannot be created for Tasks and Events.