Tami,
You can get up to 10 phone numbers for each Skype account, in any area code you like (and in some cases abroad) which is nice b/c if you are on the west coast and have clients on the east they can call you for free.
I'm not sure about the 1-800 set up on Skype but am assuming that you can have your 1-800 number forwarded to ring to your Skype number (once you've paid for it that is) (you can also program your Skype numbers to forward to say your home line or cell.)
When you sign up for SkypeIn you pay for a phone number on Skype and it comes with free voicemail. You then use that number to allow clients or whoever to call you just like they would on a regular phone. I'd just pick up and dial the number, your Skype would ring.
You can call ANY other Skype members for free on Skype at any time with or without a SkypeIn number, and with or without SkypeOut credits.
SkypeOut credits are purchased 'minutes' that you can use to call OUT to any number on the planet (we use Skype to call family and friends the world over and it's CHEAP!) The money you pay into SkypeOut ticks down in your account as you use it for calls, costing more for long distance - the same as a regular phone but much cheaper.
I do not have Skype Pro and don't know what the difference is but we've been just fine with our regular Skype account. If Skype Pro gives you unlimited US calling and you do alot of long distance inside the States then it's probably a great deal.
Why not download and install the Skype interface and check out, you'll learn alot about how it works just by clicking on the tabs to see the options in the program itself.
Hope that helps!