Your VRChat account can be identified by a few different factors. It is important not to get these mixed up, as it can lead to confusion and make troubleshooting difficult. We'll break down what these mean individually.
This is the name shown to other users in-app, viewable in the nameplate above your head and in menus. You can change your display name by editing your profile on the VRChat website. If you're using a Steam/Oculus/Viveport account, your display name will match your name on the corresponding service but with a four-character identifier at the end. When you change your name on the respective service, your display name will be updated to match the new one (along with the identifier) the next time you launch VRChat. To remove the identifier, see how to upgrade to a VRChat account.
This is the very first name you used when signing up. This name is only visible for yourself and other users aren't able to view it. For Steam/Oculus/Viveport accounts, this will typically match a unique account identifier on the related service, such as a SteamID. Please note that you cannot change your username.
This is a unique string of characters generated upon creation that identifies your account. It can be found in the URL of your account on the website and cannot be changed or customized.
When logging in to a VRChat account on the VRChat Application, website, or SDK, you need to enter your username or email address (not your display name), along with your password. If you're using a Steam/Oculus/Viveport account, or if your VRChat account is linked to one of those services, you can also access your account by signing in with the respective service in the VRChat Application only when launching the application via the same service.