SendGrid provides and manages an email server on your company's behalf, so your communications with customers are reliably sent and delivered as needed. It's perfect when you need to send transactional emails, password recovery emails or notifications to users from your website.
Segment is a Customer Data Platform (CDP). They provide a service that simplifies collecting and using data from the users of your digital properties (websites, apps, etc). With Segment, you can collect, transform, send, and archive your first-party customer data.
Zendesk is a customer support tool. It's got 2 main features: ticketing and knowledge base. Ticketing: Zendesk gathers customer queries in one place where you can manage and respond to customer questions efficiently. Knowledge base allows you to create an FAQ section to guide your customers.