Here is what Wikipedia has to say :-

Every company that sells its products and/or services to customers has functions responsible for managing the customer fulfillment and relations.

In traditional businesses, those functions are referred to most commonly as "Fulfillment", "Post Sales" or "Professional Services".

Cloud Service is a method of enabling a software solution to customers in a subscription model that departs from the "old" model of
granting a perpetual license that enables the customer to own the solution and therefore use it as they see fit (but also be responsible for its operation).

Rather, when enabling a solution to customers as a Cloud Service, companies offer their products as services instead of physical objects, moving the
economy to a subscription model.

The customer "rents" the solution and is able to use it only for the period they have rented it for.
The vendor enabling the solution provides not only the solution itself, but also the infrastructure that supports it.

The key implication of this new model is a fundamental shift in the engagement model between the software vendor and its customers.

If in the traditional "enterprise software" model, a customer buys the license to the software and pays the vendor then, regardless of its actual usage,
in a Cloud Service model, the customer pays a (much smaller) rent for the software every month.

The software vendor must therefore ensure the customer is using the solution and seeing value from it if they wanted to ensure the customer continues to
pay their rent.

This fundamental shift in the software industry's operating model revealed a need for a function at the company to own and ensure that success of its customers.

The emergence of this function is what is now being referred to as customer success (CS).
While the trend towards Cloud Service has been going on since the beginning of the 21st century,[12] the understanding of the need for much stronger focus on
customer success and therefore the creation of the field of customer success only began around 2010�2012.

On LinkedIn, the world's largest repository of professional bios, only a very small number of people had titles that included "customer success" in 2010 or 2012.

By 2015, this number was huge and growing rapidly.

The reason is that the impact of customer success on the performance and value of a Cloud Service company is huge. The CS function is responsible for retaining
and growing the business that the sales team has secured. Case studies show that companies with strong CS teams outperform peers with weak or no CS teams in a
multitude of financial criteria including customer retention (also measured by "churn", which is the opposite of retention), revenue growth rates, gross margin,
customer satisfaction, and referrals.

In fact, customer experience is the greatest untapped source of both decreased costs and increased revenue in most industries, but only if companies take the
time to understand what underpins it and how they can benefit financially from improving it.