Main applications for this project.
The directory name for each application should match the name of the executable you want to have (e.g., /cmd/myapp).
Don't put a lot of code in the application directory. If you think the code can be imported and used in other projects, then it should live in the /pkg directory. If the code is not reusable or if you don't want others to reuse it, put that code in the /internal directory. You'll be surprised what others will do, so be explicit about your intentions!
It's common to have a small main function that imports and invokes the code from the /internal and /pkg directories and nothing else.
See the /cmd directory for examples.
Private application and library code. This is the code you don't want others importing in their applications or libraries. Note that this layout pattern is enforced by the Go compiler itself. See the Go 1.4 release notes for more details. Note that you are not limited to the top level internal directory. You can have more than one internal directory at any level of your project tree.
You can optionally add a bit of extra structure to your internal packages to separate your shared and non-shared internal code. It's not required (especially for smaller projects), but it's nice to have visual clues showing the intended package use. Your actual application code can go in the /internal/app directory (e.g., /internal/app/myapp) and the code shared by those apps in the /internal/pkg directory (e.g., /internal/pkg/myprivlib).
Library code that's ok to use by external applications (e.g., /pkg/mypubliclib). Other projects will import these libraries expecting them to work, so think twice before you put something here :-) Note that the internal directory is a better way to ensure your private packages are not importable because it's enforced by Go. The /pkg directory is still a good way to explicitly communicate that the code in that directory is safe for use by others. The I'll take pkg over internal blog post by Travis Jeffery provides a good overview of the pkg and internal directories and when it might make sense to use them.
It's also a way to group Go code in one place when your root directory contains lots of non-Go components and directories making it easier to run various Go tools (as mentioned in these talks: Best Practices for Industrial Programming from GopherCon EU 2018, GopherCon 2018: Kat Zien - How Do You Structure Your Go Apps and GoLab 2018 - Massimiliano Pippi - Project layout patterns in Go).
See the /pkg directory if you want to see which popular Go repos use this project layout pattern. This is a common layout pattern, but it's not universally accepted and some in the Go community don't recommend it.
It's ok not to use it if your app project is really small and where an extra level of nesting doesn't add much value (unless you really want to :-)). Think about it when it's getting big enough and your root directory gets pretty busy (especially if you have a lot of non-Go app components).
The pkg directory origins: The old Go source code used to use pkg for its packages and then various Go projects in the community started copying the pattern (see this Brad Fitzpatrick's tweet for more context).
Application dependencies (managed manually or by your favorite dependency management tool like the new built-in Go Modules feature). The go mod vendor command will create the /vendor directory for you. Note that you might need to add the -mod=vendor flag to your go build command if you are not using Go 1.14 where it's on by default.
Don't commit your application dependencies if you are building a library.
Note that since 1.13 Go also enabled the module proxy feature (using https://proxy.golang.org as their module proxy server by default). Read more about it here to see if it fits all of your requirements and constraints. If it does, then you won't need the vendor directory at all.
OpenAPI/Swagger specs, JSON schema files, protocol definition files.
See the /api directory for examples.
Web application specific components: static web assets, server side templates and SPAs.
Configuration file templates or default configs.
Put your confd or consul-template template files here.
System init (systemd, upstart, sysv) and process manager/supervisor (runit, supervisord) configs.
Scripts to perform various build, install, analysis, etc operations.
These scripts keep the root level Makefile small and simple (e.g., https://github.com/hashicorp/terraform/blob/master/Makefile).
See the /scripts directory for examples.
Packaging and Continuous Integration.
Put your cloud (AMI), container (Docker), OS (deb, rpm, pkg) package configurations and scripts in the /build/package directory.
Put your CI (travis, circle, drone) configurations and scripts in the /build/ci directory. Note that some of the CI tools (e.g., Travis CI) are very picky about the location of their config files. Try putting the config files in the /build/ci directory linking them to the location where the CI tools expect them (when possible).
IaaS, PaaS, system and container orchestration deployment configurations and templates (docker-compose, kubernetes/helm, mesos, terraform, bosh). Note that in some repos (especially apps deployed with kubernetes) this directory is called /deploy.
Additional external test apps and test data. Feel free to structure the /test directory anyway you want. For bigger projects it makes sense to have a data subdirectory. For example, you can have /test/data or /test/testdata if you need Go to ignore what's in that directory. Note that Go will also ignore directories or files that begin with "." or "_", so you have more flexibility in terms of how you name your test data directory.
See the /test directory for examples.
Design and user documents (in addition to your godoc generated documentation).
See the /docs directory for examples.
Supporting tools for this project. Note that these tools can import code from the /pkg and /internal directories.
See the /tools directory for examples.
Examples for your applications and/or public libraries.
See the /examples directory for examples.
External helper tools, forked code and other 3rd party utilities (e.g., Swagger UI).
Git hooks.
Other assets to go along with your repository (images, logos, etc).
This is the place to put your project's website data if you are not using GitHub pages.
See the /website directory for examples.