What is lead process/Sales process/Support process in Salesforce?

A sales process is used to determine which opportunity stages are selectable when record types are enabled.  A sales process is not required if record types are not enabled.

Every company is unique, but all companies want to find, sell to, and keep customers. Salesforce has the tools you need to grow your pipeline and make more sales.

Salesforce features designed to support your sales process include leads, campaigns, products, pricebooks, opportunities, and quotes. In this section, we’ll learn about leads and opportunities. You can use opportunities alone, or you can enhance your opportunities by using them along with leads.

Setting up a sales process:

You might sell different items using different processes. You must set up at least one sales process in Salesforce, but you can set up additional processes to match how your business actually works.From Setup, click Customize | Opportunities | Sales Processes.
Click New.
To create the first new process for your organization, create a Master process.
Give your sales process a unique but descriptive name. For example, Retail Sales.
Optionally, add a description.
By default, all stages are included in a new process. Remove stages you don’t want to include by clicking the stage name in the Selected Values area, and then clicking the Remove arrow to move the unwanted stage to the Available Values area. For example, if you qualify all leads before converting leads to opportunities, remove Qualification from the selected values.
Click Save.

Leads Process

Every contact you get from these campaigns is added to the system as a lead. A lead is basically a business card with a name, address and phone number that could potentially be interested in your product. You either acquire this card in person, mail or lists. You have activities related to the lead development which will include calls, sending materials and possible meetings. A lead has a life cycle with lead stages and status. As a lead moves through the life cycle you can capture the time it took to move from one stage to another. Lead scoring also plays a huge part in prioritizing lead management.
There is a lot of debate on when is it right to convert a lead to an Account, Contact and Opportunity. I always advise on converting a lead when there is even a 10% chance of getting business from that lead.