If you use ChannelCRM to manage your sales, then there are two important concepts: Opportunities and Quotes.
Opportunity is the internal recording of something you think you could sell to a prospect.
A Quote is always linked to an opportunity in the sense that the quote is what you show / send to the customer.
An opportunity may well have more quotes linked, because you may have to make revised offers (quotes). The lead may want a different composition of products and amounts - or you may have to adjust the price.
But there can be only one active quote for an opportunity. The other quotes will then be marked as history.
To make a quote is easy when there is an opportunity in advance: You click either "Create quote" or you just merge to a document (letter, email, ...). The rest is usually carried out automatically.
Opportunities are records of anything you think you could sell to a topic. An opportunity can be quite simple: "I can sell for 100,000 to ProspectA". But it can also be more complete, including product lines with products, prices, discounts and so on. A complete registration is required to make a quote (offer) to a customer.
Many start by creating a sales opportunity as simple as possible (lead and amount) and when you have uncovered the needs of the prospect, you make changes to the opportunity to be complete with products and product lines.
With the opportunity comes a probability. Either you can set this yourself (eg. 40% if you think that there is a 40% probability that the order comes in). Or you let a Sales Model dictate the probability based on how far you are in the sales cycle.
Here is an example of an opportunity: