In the U.S., patent numbers use a continuous numbering system. This means that for each type of patent, the numbering starts at one and increases by one as each new patent is awarded. In the U.S., patent numbers are seven characters long, so the very first utility patent awarded, for example, would be 0000001. You can tell the year a patent was issued based solely on its number. For example, a patent number between 110,617 and 122,303 was issued in 1871. If you're searching for the patent outside of the USPTO database, you'll also need to add the prefix US to the number to indicate it is a United States patent.
Patent numbers that don't have a letter in them (excluding the US prefix if you're searching outside of the USPTO database) are utility patents. A utility patent is the most common type of patent issued by the USPTO. It refers to an invention that can be categorized as a process, composition of materials, machine or an improvement on any of these categories. A utility patent gives the inventor exclusive rights to make or sell the invention for 20 years.
A patent number that starts with a D is a design patent. These are a little more complicated, but are essentially ornamental designs added to a tangible item. To qualify, the design must be original, new and can't have any useful purpose that might qualify it for a utility patent instead. The design must be fixed to the item and can't be removable. A design patent protects the appearance of an item, while a utility patent protects how the item works or how it's used.
A patent number that starts with the letters PP indicates that it's a plant patent. Any plant that qualifies must be asexually reproduced and must be newly invented or newly discovered. The plant can't be in an uncultivated state. The USPTO website has more details regarding what types of plants qualify. A patent award grants the owner an exclusive right to make the plant, use it or sell it for 20 years.
An RE before a patent number indicated it is a reissue patent that corrects an error in a previously awarded patent. A T indicates it's a defensive publication, meaning it was published to prevent others from obtaining a patent but the inventor didn't seek patent rights. An H means statutory invention registration, which is the same as a defensive publication, but for the years 1985-1986 only. An RX before the patent number means the patent was re-examined to validate it based on another party's request. And an AI before the patent number stands for additional improvement, a designation given from 1838 to 1861 for patents in which an inventor was improving on his own invention.