Hi Zanester & welcome to the forums :)
Sorry for being pedantic... but it's possible to load Samplers and soft synths on Software Instrument tracks / Channels.
The difference is basically a Sampler uses audio samples in some way mapped to Keys on a MIDI keyboard. A sampler often then has to load these samples into your computers RAM. Logic's Sampler is the EXS24 (but Ultrabeat can also act as a sampler).
A synth is basically generating sound from scratch using osciallators... often this is less CPU intensive.... depends...
Anyway, that was hardly a warm welcome! Sorry ;-)
Imagine Logic is like a real studio setup - the Enviroment is the map of the entire studio.
So the Mixer and Arrange area are the Mixing desk etc... all tidy and easy to get to etc. Your hardware Compressor and reverb units are the effects....
The Environment is all of that plus all the cables and patch bays underneath the mixing desk that connects audio channels and MIDI equipment and all that stuff that means you can record, monitor, play around with other devices and incorporate them into your studio setup.
So, the Environment allows you to go a little deeper and customise signal routings and faders and knobs and create controls that can control multiple parameters and create arpeggiators and much more.
The beauty of Logic 8 and 9 is that you rarely need to go to the Environment at all if you don't want to. However, the Environment can be a very creative place.
It won't necessarily be the place to go to save CPU... that might come in the form of bouncing or freezing Software Instrument tracks to audio or using Aux channels for effects (e.g. one instance of reverb) and sending from multiple channel strips to that Aux channel rather than inserting a reverb on every channel.
I'd really recommend checking out the Core Logic 101 - which contains so many indepth tips on everything you need to get into Logic... But of course, do let us know if you have any further questions - I'll try and keep my answers a bit shorter next time!