There are many subtopics under the heading US Postage Stamps - everything from the most recent issues to the classics of US philately. Breaking the topic up into a few key categories makes it easier to tackle...and makes finding the part you're interested in a little easier, I hope!
1. Rare US Stamps
Everyone has their own definition of rare, but I'm thinking of material worthy to be sold in the high-end stamp auctions, such as the annual "Rarities of the World" sale held by Robert A. Siegel Auction Galleries.
2. Collectible US Postage Stamps
Everything from the rare to the common can be included depending on your definition of collectible. So there can definitely be a bit of overlap between this category and some of the others.
But that's ok; one of the great things about stamp collecting is that you can go about it however works best for you and classify your stamp collection any way you want.
3. US Postage
Most mint (unused) stamps issued since about 1940 are in this category. These stamps are so plentiful that many people use them as postage for mail.
The stamp collecting boom of the 1930s led to collectors and non-collectors alike to begin hoarding mint stamps as investments. So many millions of these stamps were produced and saved that most have a resale value that is lower than their face value.
The net result is that, with few exceptions, most US stamps printed in the past 75 years can safely be used on envelopes.
I always use discount postage to save a little money. It really adds up if you do much mailing. For just one priority mail package, discount postage can save about $2, likely lower if you find a good deal.
You can buy discount postage that is valid for mailing from most stamp dealers and can usually buy it anywhere from 70 to 90% of the face value depending on which denominations of US postage stamps you buy.
You should be able to sell forever postage stamps in the 80 to 90% of face value range and maybe even a little higher with a bit of research and legwork.
This is a special category unto itself.
Think piles and piles of used US postage stamps issued by the USPS over the past 75 to 100 years.
Modern US Used stamps are virtually worthless but where else can you find a hobby that will cost you almost nothing?
I usually donate this type of material to veteran's or children's charities but you may be able to find a buyer that will pay a few dollars per pound of stamps.
Of course, this usually has to be an in person purchase because the cost to ship the stamps is often more than they're worth.
5. Postal History
Envelopes with stamps and markings intact are generally described as postal history.
This category of collecting has grown over the past few decades as more and more collectors take an interest in the entire mailing process - from searching for rare and unusual postal markings to hunting for specific stamps on cover.
"Cover" is a common collector term for an envelope that was passed through the mail system...as in an the cover or wrapper around a letter.
6. First Day Covers (FDCs)
A specialized area of cover collecting focused on envelopes with stamps used on their first day of issue by the postal service.
Now a days there are many artists that specialize in preparing collectible envelope cachets and artwork to be used on first day covers for new issues. These are collectible more for the artwork than for the stamp as new stamps are common.
Of course, as you go back further into the past, first day covers become exceedingly scarce.
When you get back into the 1890s, for example with the famous 1893 Columbian Exposition Issue celebrating the 400th anniversary of the discovery of America, only a handful of first day covers are known and are valued at tens of thousands of dollars each.
Also known as personalized postage stamps, you choose a picture to use on a stamp for mailing through the US Postal Office.
These custom postage stamps are produced by companies licenced for business by the USPS.
You have to pay more per stamp than the standard postage rate but these are pretty popular for anyone that wants to personalize their mail - kids and pets are especially popular subject matter.
There is a ton of stuff you'll find in US stamp collections that doesn't really fall into the above categories.
This can be libraries of reference materials and philatelic literature, Cinderellas, or pretty much any other form and format you can imagine.