What are services in OS X Server?
The Services are the different jobs that the server can do for you and anyone else who uses the server (assuming they have their own computer and a valid password to the server). Many/most of the Services are intended to help groups of people work together. Many/most of the Services can be set up so that people with different OS can work together.

Usually, you won't need or enable all of them but it's worth considering what you might make good use of and experimenting with some of them.

Below is a list of descriptions of the Services that I've shamelessly plagiarized from the book: OS X Server Essentials 10.10 by Arek Dreyer &amp; Ben Greisler (I have a hard copy but you can buy it via itunes and other fine stores - https://itunes.apple.com/book/os-x-server-essentials-10.10/).

Caching - speed up the download of software from Apple

Calendar - share calendars, book resources like meeting rooms, projectors, etc

Contacts - share Contacts information

Filesharing - share files. These mount like hard drives on the user's computer. 

Mail - provide email service (needs a domain name, obviously)

Messages - used to be ichat. Think of it like Skype. Users can text each other, video conference, etc.

Profile Manager - manage the different Apple devices you are responsible for from the server

Time Machine - backup the different Macs on your network to the server

VPN - users not physically at the same location as the server can connect to it over the internet and be treated as if they are physically there (so they can use printers, other services, etc).

Websites - a web server

Wiki - a web site that can be easily edited with free-form information. Think Wikipedia but not as sophisticated. Each user can have their own wiki.

Xcode - allows developers of Xcode projects to share code and collaboratively build something