Envelopes give the ability to control how a sound reacts over time. They determine how fast or slow the sound starts, the volume at which the sound stays at while holding a key, and how fast the sound dies away after you release the key.
The Wavestation basically has two envelopes that you can use, one that affects the oscillator volume, and an assignable one that you can route to a number of different functions. Both of these envelopes work on a per-oscillator basis, so in a four oscillator patch you can have a seperate amplitude envelope for each oscillator, with different settings. The same goes for the assignable one as well.
If you want your sound to fade out over time, you'll create one with greater amplitude at the beginning than at the end. If you want the fade out to happen slowly you'll use longer times and if you want it to fade out quickly you'll use shorter times.
If you don't care to experiment with the volume ("amplitude") envelope, the Wavestation offers a few defaults from which you may choose.
The basic filter section of the Wavestation is quite simple. It consists of a Cutoff (as in "at what percentage do you want the filter to cut off the sound") setting; a keyboard tracking and an exciter. The filter can be modulated by all of the usual modulation routings. You may find that it's fun to modulate the filter with Env1 (the assignable envelope) or LFO2.
Low Frequency Oscillators can be used to modulate any number of parameters. The traditional use of an LFO is to create vibrato in a sound. In other words, it is often used to modulate the pitch of a sound -- in more or less subtle ways.
The Wavestation has two flexible LFO's that allow you to adjust how fast they oscillate (rate), how much (depth) they oscillate, whether the oscillation will have a fade-in or delay before it occurs after note-on, and you can modulate the rate and depth with other sources (where you play on the keyboard, for example).