Salesforce is a cloud based Customer Relationship Management company based out in San Francisco USA. The company was founded in year 1999 by Marc Benioff , who is the current chairman and co- CEO.
The idea behind salesforce is to provide the CRM system as a subscription service. Gone are those days where customers need to build an infrastructure from scratch & wait long time to release a most viable product. Salesforce provide a generic prebuilt CRM system which can be used by the customers with minimal customization. Let say a company wants to have a CRM system to manage customer relationship, they can call salesforce and purchase necessary user licenses and then can start using the prebuilt generic CRM system right away with minimal setup from there end. It is very easy with the subscription model to ramp up/ramp down the users as per business needs. Obviously, your salesforce instance can also be configured to make it complaint with your company policies & Non functional requirements.
Salesforce provide SAAS(Software as a service) and PAAS (Platform as a service). Under SAAS , they provide various CRM products like Sales cloud , Service cloud etc. With PAAS , they have built an ecosystem for various partners to build third party products which can be easily integrated with CRM products or can be used on there own. Customers can also build custom solution or extended the capabilities of standard CRM products according to there needs. One of the important features of Salesforce is less code as they provide lot of point and click / declarative tools to build automations. You can definitely build complex apps using code as well.