Laptops range from a 13 inch to a 17.3 inch screen typically and the price range for mid-range is usually anywhere from 300-700 dollars. They can weigh from 3-10 lbs but on average they are 5 lbs.They have the most power out of the 3 to handle applications and have faster processing for photo/video editing. They have the most harddrive space to store Music, Documents, Videos, Photos etc.
Netbooks are great for quick light weight travel. They are mainly cheap and have around an 8-11 inch screen. They are portable and can handle simple applications such as web browsing, Office productivity programs, viewing photos and playing some flash games. Most netbooks have a long battery life and can be used on average for up to 8 hours.
tablets are light-weight and are fairly new to the market and as such are more expensive. The operating system is arguably easier to use because it is based on simple touch gestures and applications are easily accessible. Their processing power is fair, but some tablets cannot use Flash *cough* IPad* cough*.However along with the down-side of price, they can't store too many videos, documents, music or w/e and have around an 8-32 gig harddrive space, If you plan on spending more for harddrive space I'd recommend getting a laptop.
As for your question on smart phones related to tablets, tablets can be connected to the internet all the time if you
1. Buy a tablet that has a 3G option
2. purchase a 3G data plan from a phone service that is compatible with the tablet(ATT goes with IPad, Verizon goes with Blackberry Playbook)
you don't need a card for it, as it is built into the tablet. Just make sure to pay for the plan to access it.
If you don't really want that feature for yourself, you can get a tablet with a Wi-Fi only option that is cheaper, but you can only connect to the internet via a hotspot in your home or a Mcdonalds, starbucks, Cafe etc.
If you do really want a tablet though, you should wait another 2-3 years for the price to drop down or for more options to be available on the market. at the moment I can only recall 3 tablets off the top of my head and none of them I would want to buy now. I'd especially avoid the Ipad 2 because it really doesn't have too many more improvements than the Ipad 1.