What is "Third Party Grading" when it comes to Sports Cards?

i get this question a lot.  
The gist of it is that they are companies that provide their "expert" opinions on what they view the condition of the card(s) to be. 
There are three Third Party Graders (TPG) worth bothering with.  There are also a bunch of others, and seemingly more pop up from time to time to try to take advantage of people. 
BGS/BVG, PSA, SGC
Now, everyone has their preferences, and thats fine, as there is no "wrong" way to collect.   Here are, IMO the Pros and Cons of each company:

BGS/BVG
Pros:
- Fantastic Protection for the card, easily the "best" of the TPG.
- They do not require membership to submit your cards.
- They provide Sub Grades (Centering/Edges/Corners/Surface) for each card they grade.
- STRONG Modern Release Presence.  They are the "go to" TPG for Modern Cards
Cons:
- for cards prior to 1981, BGS becomes BVG (Beckett Vintage Grading) and Sub Grades are not provided
- While they scale from 1 - 10, with half point grades, they use different color/material Flips (the slip showing the card description and grade)
Grade 1.0 through 8.0 = White, paper flip
Grade 8.5 & 9.0 = Silver, metalic flip
Grade 9.5 & 10 = Gold, metalic flip
i feel this can be seen as "punishment" for the owner of the card.... oh, it's a lower grade? "F" You, heres an inferior flip because of it. This
also creates a visual eye-sore when cards of various grades are displayed together imo.
- BGS/BVG Slabs take up a lot more room vs the other two.

PSA
Pros:
- Longest Presence in the hobby.  Started in 1991.
- The power of their Registry Program cannot be denied.
- Strongest following in the overall hobby, especially in post-war through the 1990s due to their Registry Program.
- They also grade from 1 - 10 with half point grades, but unlike BGS, they use one color/material for all their flips, this leads to amazing
eye-appeal when cards of various grades are laid out together.
Cons:
- Membership is required for card submission
- Lack of Sub Grades is a turn-off to some.
- Slabs are considered by many to be "flimsy"

SGC
Pros:
- They are very good with pre-war cards
- They require no membership for submission
- They have a black "frame" that surrounds the card
Cons:
- They use a numbering system that i have still not memorized, using a 1 - 100 scale, with various skipping depending on the strength of the cards grade.
- They have a black "frame" that surrounds the card

SO, which TPG should you use?  Of course there is no clear cut answer.  Use what YOU prefer!  As mentioned, each has their Pros & Cons.
NONE of them are "Perfect", and thats ok, because thats how it goes.   I own lots of slabs from all three, and there have been times that ive
gotten in a slab and said "WTF? how did it get (grade here)?"
i have friends that only collect PSA, some that only collect BGS and some that collect from all three.  

I've got gobs and gobs of worthless slabs of players that never panned out to anything.  It wasn't until 2008 that I started taking a serious
look at the players i was collecting.  it was then that i put my foot on the gas pedal and started seriously chasing the PSA Post War HOF RC Set,
which i i was fortunate enough to "complete" at the show last year.

With that completed, I am in a bit of a FOCUS limbo, going here and there, but it really does always come back to Rookie Cards for me. 
i love rookie cards, always have, always will.  i guess its just how it goes for collectors my age and how things were when we were young.

i am always willing to help out my fellow collectors however i can, please feel free to ask anything you care to.

Thanks Again for the read,
Eric