For new users
For people who are new to HIVE it can be a very intimidating place. It is NOT intuitive and the whole Resource Credits, Hive Keys, Username, and multiple onboarding pathways make things difficult.
If you add the fact that not all onboarding processes work the same way it can get even more confusing. Indeed when I joined Splinterlands I had no idea that my Splinterlands account was also my everything else on HIVE account as well. That's why I have both a account for Splinterlands and I made a
account to access Hive-Engine only to find out I should have just used the same account for both.
The one username and password for multiple sites confuses people who are used to every website having its own username and password combination.
This is primarily directed at 
She started with an account with Ecency but didn't realize how important her keys were and lost them. Then I made an account for her that she couldn't access. Then she got a free account and was worried it will be limited because its free.
Now when I started with Splinterlands I paid for a "Spellbook" but what I didn't realize is that the Spellbook was actually a Hive account AND that the payment helped to fund the account with Resource Credits so I could realistically post as much as I liked. The account was free and as such had no Resource Credits associated with it. That means I was continually running into errors that made sense when I was new. Things wouldn't post and upvotes wouldn't work. Very annoying as I thought I was having a faulty account.
So here is how the accounts work.
There are no free accounts
Now you may disagree with that statement because some accounts are made available for free but HIVE requires payment for new accounts. The payment can be made in two ways.
- Three Hive can be spent to make an account
If you have three HIVE you can make an account. I think that it hard coded into the HIVE architecture but I could be wrong. However HIVE have a dollar value attached to them. Burning those HIVE to make an account has a cost so some websites pass those costs along to people to make accounts for them. Some also add a bit of markup in there
OR
- A whopping number of Resource Credits can be be spent to make an account
When the account had about 9000 HP through its own and delegated HP it could "spend" resource credits to get Hive account creation tokens. Over a few months the account "bought" about 45 username tokens. True there was no dollar cost associated with the purchase but the resource credits only accumulate so quickly. Making a Hive username took me almost 3 days and then the account was depleted.
Some larger accounts will provide those accounts to new users for free. Unfortunately they are in limited quantity because no account and provide an unlimited number of new user accounts for free. Either spending HIVE or RC (which means time) is required.
All accounts are created equal..mostly
Every account that is created works exactly the same way and has the same abilities. However, account "abilities" cost resource credits. If you don't have enough resource credits you can't perform actions.
As an example has an account with only the 10 HP that I delegated to her. While her account and keys have the option to make new usernames she doesn't have nearly enough HP / Resource credits to do that action... yet :)
But I say mostly for a reason.
There are HIVE actions such as posting, delegating, curation and such who are linked directly to the HIVE username and keys.
However, certain apps built on top of HIVE have additional commands and features built into their apps. For example: Actifit has an additional password added for their app/website which allows for burning AFIT tokens and moving AFIT tokens onto and off of the HIVE blockchain. As it is not a native HIVE command a basic username won't have that ability unless it is signed up to Actifit.
Splinterlands would be another example. A Hive username will allow you to user the Splinterlands website which is based on HIVE no problem. However, actually getting rewards and doing battles requires a purchase from Splinterlands of a "Spellbook" which unlocks their game system.
How should I sum it up?
I think of it this way. There is a base level of HIVE. Username and keys interact with the base level of hive the same way.
Apps can be built on top of that layer. The creators can add additional features or restrictions in their app sitting on top of Hive.
Take home message for new user
HIVE usernames don't have "tiers" or PAID vs FREE restrictions.
If you have a HIVE username and all four keys associated with you then you are an equal citizen on HIVE. As you grow your HP stash your votes will become more valuable and as you grow your reputation you will be more respected but no account is built better or stronger than any other.
As for branch out into other applications on HIVE you will find that there is a rich ecosystem of different things to do with your account. It doesn't make your username and keys any less powerful but it does add another layer of benefit and possible complexity.
But when you are new? Just know that everyone else has the same issues when they start out. In time it makes sense but not from day 1 :)