Lifecyle of sales, marketing and service modules in Dynamics CRM

Sales:- It's a process of converting new leads converting them into customers(contact,account) and working on opportunity against them. opportunity itself can have stages.
While in opporunity you can go to create quotes, orders and invoices for the sales of your product.
 

Marketing:- As the name says it is the marketing of your product,services etc. You can create Marketing list for leads/account/contact and distribute the marketing activities among them in a marketing campaign. Remember each campaign has a plan, a buggeted cost. The result of these marketing activities can result into the new opportunites with existing  customers or new leads.
So the output of marketing process is basically input to the sales process.

Service:- This is the 3rd module in MS CRM which is normally used if your company is providing services for your product like maintence service or other services. You can have contract against your customer which can define how the customer are serviced (i.e. cases).