The image below is of a Thai visa. It is issued outside of Thailand, by a Thai diplomatic post operated by the Thai Ministry if Foreign Affairs. It is a multi-color paste-in decal.
The red arrow points to the Class of the visa, which may be: B (Business); O (Retirement or person with Thai family member); E (Education); T (Tourist); TR (Transit).
The blue arrow points to where is shows whether the visa is for S (Single) or M (Multiple) entry.
The purple arrow points to the "Valid until" date, after which the visa may no longer be used.
The yellow arrow points toward the traveller's passport number
Except when you have a multiple entry visa, once you enter Thailand, the visa will be annotated "Used," along with the machine-stamped date of your entry stamped diagonally across the visa, and that visa will be cancelled.
There is no such thing as "extending a visa." Once a visa is stamped "USED", or its "Valid Until" date is reached, it is dead and no longer valid.
An initial entry permit is a bluish rectangular ink stamp paced into your passport by an Immigration officer of the Royal Thai Police, at an entry point into Thailand. It can normally be issued for 15, 30, 60, or 90 days, depending on the type of visa that was in your passport when you entered Thailand.
It normally has a mechanically-stamped issue date (indicated by red arrow), and a hand-written "Admitted until" date (indicated by blue arrow). It also normally has a hand-written visa class, which is often difficult to read, and which is based on the class of your visa when you entered Thailand (indicated by purple arrow):
If you apply for a long-term extension of your entry permit, you will normally be issued a purple or bluish-green ink "Under Consideration" stamp, stating that your "Application for extension of stay is under consideration". It is normally issued for 30 days. It has the date by which your passport must be returned to Immigration (indicated by red arrow), and also the date of issue (indicated by blue arrow), with both dates normally machine stamped
Once you receive an entry permit extension, you will receive a blue-ink stamp that says "Extension of stay granted until DATE" with a machine stamped expiration date (indicated below by red arrow), an issue date (indicated by blue arrow). There is also a hand-written Immigration file number for your case (indicated by purple arrow)
For any entry permit, in the event that you will exit Thailand before the expiration date of your entry permit, and you wish to reenter with that entry permit still remaining valid, you must obtain a reentry permit from an Immigration office. A re-entry permit is a large blue or black ink-stamp that is placed into your passport. It has a "Valid Until" Date that is normally machine-stamped, and which matches the expiration date of your underlying entry permit (indicated by red arrow), the number of reentries that the stamp is valid for: Single or Multiple (indicated by blue arrow), and the date of issue (indicated by purple arrow)
Another kind of stamp that Thailand will routinely place into your passport is a departure stamp, which is normally a small, triangular, blue ink stamp, containing machine-printed departure date:
Two other types of stamps are a penalty stamp for late submission of the required 90-day report of address:
and the penalty stamp that you will receive if you overstay beyond the "Admitted until" date of an entry permit: