Connection -- LinkedIn user you have chosen to connect with, allowing each of you to view the other’s profile and network with each other’s connections; think ‘friend’ on Facebook combined with Six Degrees of Separation; breaks down into three tiers: 1st-degree, 2nd-degree and 3rd-degree

1st-degree connection -- users who have accepted your invitation to connect, or users whose invitation to connect you have accepted; these are likely people you have met in the real world through business, school, etc. or otherwise have some direct connection with outside of LinkedIn; can be contacted via a message on LinkedIn

2nd-degree connection -- the connections of your 1st-degree connections, a ‘friend of a friend’ so-to-speak; can be sent invitations to connect or be contacted through an InMail or an introduction

3rd-degree connection -- the connections of your 2nd-degree connections, a ‘friend of a friend of a friend’ so-to-speak;
can be sent invitations to connect if their first and last name are displayed; if only the first letter of their last name is displayed, can be contacted through an InMail or an introduction (invitation to connect is not an option)

InMail -- messages you can send directly to another user, one you are not connected to; can be purchased or received as part of a premium account subscription

Introduction -- means of contacting users in your extended network through the people you know (your connections); to contact a LinkedIn member who is two or three degrees away from you, request an introduction through one of your connections; from there, that connection chooses whether to pass your message on to its intended target (if they are in your 2nd-degree network) or on to a shared connection (if the intended target is in your 3rd-degree network)