A locked knee gets totally stuck in a position from which it cannot be unlocked without help, while pseudolocking can be 'unlocked' just with a simple manoevre. Locking may be due to a number of things, but very often it is due to a bucket-handle tear of the meniscus ('cartilages'), where a bit of torn meniscus flicks itself over and gets stuck between the bones of the femur (thighbone) and tibia (shinbone). Giving way may also be due to a meniscal tear (and other things), but is a common complaint when the ACL is torn.
The combination of an ACL and a meniscal tear is very common - they may both occur at the same injury, or the meniscal tear may be a consequence of the disordered mechanics of the knee after an ACL tear.
Pseudo-locking and giving way can also be features of mal-tracking and plicae, but the symptoms in this case of of a lesser nature.
You really need a really competent clinical history taken (your story) and a thorough examination to assess what is happening, but it seems pretty certain that there is some internal derangement.