The Jira Slack Integration provides bi-directional communication between the Jira Cloud instance and the Slack. 
You can find this app on the Atlassian Marketplace here: https://marketplace.atlassian.com/plugins/jiragitintegration/cloud/overview. 

Any change, comment or update within Jira can be configured to notify to one or multiple slack channels.
A slack user can participate in a discussion around a Jira issue and that conversation is sent to Jira.   

Notifications

The Jira Slack Integration addon allows you to create notifications at three levels within Jira.  At a system level, a project level and an issue level.

System Notifications

System notification work at a system level meaning any notification set up here responds to events within the whole Jira system and include all projects and all issues.   A user can create 1 or many notifications at a system level.  System level notifications can be created or configured at the System Level administration screen.  

Issue level notifications are shown here but can not be created here.   

For more information please see Creating a System Notifications

Project Notifications

Project notifications work at a project level meaning any notification set up here responds to only events for that specific Jira project including all issue associated with the project. A user can create 1 or many notifications at a project level. Project level notifications can be created or configured at the Project Level Administration screen.

Issue level notifications are also shown and can be configured here.

For more information please see Creating a Project Notifications

Issue Notifications

Issue notifications work at an issue level meaning any notification set up here responds to only event within a specific issue.  A user can create 1 or many notifications at a issue level.  

Issues level notifications can be created or configured on an Jira Issue Page.

For more information please see Creating an Issue Notifications

Slack Slash commands

This add-on supports a variety of slash commands inside Slack. To get a list, inside Slack type "/jira ?" after connecting your Slack instance to this add-on. 

For more information on this please see Slack Commands

1. Log into your Jira instance as an admin.

2. Click the admin dropdown and choose Add-ons.
   The Find new add-ons screen loads.

3. Find Slack Jira Integration by Bitband via search.
   The appropriate add-on version appears in the search results.

4. Click Install.

5. Confirm add-on installation by clicking Accept & install.
   Now you're able to connect to Slack.

Please note that only an Jira Administrator can Install this addon as this requires access to system preferences.

1. Log into your Jira instance as an admin.

2. Navigate to the Slack Jira Integration addon page and click on the “Get it now” button.

   Open

   

3. If you have multiple Jira sites with your account select the site you wish to install the addon.

4. Select the  “Install” button to install the Slack Jira Integration.
   

5. Follow the prompts to install this addon.   

Once you have installed the addon you will need to connect to your instance of Slack.  Please note that only a Jira / Slack Administrator should do this connection, otherwise functionality in this app will be limited. 

1. Log into your Jira instance as an admin.

2. From the Jira Dashboard click on Settings as shown below.
   
   

   
   
   
   

3. Click on Add-ons → Manage add-ons and Find  "Slack Integration" in the list of Add-ons.  Select the Config button.
   
   

   
   
   

4. Click on the "Sign in with Slack" button shown below and follow the prompt to select and login to your instance of Slack.
   

   
   
   

5. Select appropriate Slack Team and Authorize.

6. Congratulations, now you’re able to use the Add-on. You can change your Slack account any time to any other account by disconnecting and reconnecting with a new account.
   

For more details check out this video:

You can create three levels of notifications that connect to a slack channel:

• System Notifications connecting to a Slack channel - provide event notifications for all Jira items and issues in all defined projects within your system.

• Project Notifications connecting to a Slack channel - provide event notification for all Jira items and issues within a specific project.

• Issue Notifications connecting to a Slack channel - provide event notification within a specific Jira issue.