What does stuff like USSJ and USS mean because I've been seeing stuff like..

There is much confusion over this topic since these are fan-made terms. This answer is not completely definitive, but here is my answer:

The dub calls it "Ascended Super Saiyan", or ASSJ. The 'U' in your question refers to the word Ultimate/Ultra. There is no name for this form in the original Japanese show. The problem with this form is speed is sacrificed for strength. There is no speed increase with the strength increase, thus making the character slower. Gokuu realized the problem with the form in the Hyperbolic Time Chamber and decided it would be used as a last resort. Vegeta also realized this. However, Future Trunks did not. He went into battle with Perfect Cell (I think) and transformed. This made him slow, too slow to beat Cell's awesome speed. Gokuu also suspected that there was a RIGHT way to "ascend" to the next stage. Gohan was the first to "ascend" properly (SSJ2 rather than USSJ/ASSJ) when he defeated Perfect Cell. Gokuu and Vegeta became SSJ2 eventually, though there is some controversy over whether Vegeta was able to become SSJ2 before the Majin Buu Saga (where he became Majin Vegeta) or whether it came with the transformation to Majin Vegeta.

For a fun debate on the subject (with pictures to start it off), check out this forum: http://forum.esforces.com/

USS is Ultra Super Saiyan which is the form Vegeta and Trunks take in the Perfect Cell Saga, but I don't know why people put a J at the end of SS. I think it's because they pronounce it as Super Sai-jin.

It stands for something like ultimate super saiyan jin ------ and ultimate super saiyan.something like that.

Well you no how ssj means supersajin well USSJ means ultimate supersajin
Plus thats how they say it in japan supersajin in america it's USS ultimate super sayin and so I hope this helps you out

Well in the Majin Buu saga when Trunks gets the dragon radar Goku slowes down Babidi and Buu before they destroy Capsule corp. And then Goku turns into an SS3 to fight Buu but Buu was confused and said su..per..sai...yjin? So basically Buu created jin wich was added on to SS to make SSj and USSJ means ultimate Super saiyan and the A means accended wich makes it Ascended Super saiyan so super saiyan is originally spelt Super Saijin.

Hope I helped