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I found a data source, where Boeing 737 could be encoded with the following values - 733, 734, 735, 738, 73G, and 73H; Boeing 767 with values 762, 763; Boeing 777 - 772, 773, 777.
Any idea why such coding is used? Is it some standard to encode them with such values? Can I get more details from such codes (for ex., full aircraft type incl. customer code like Boeing 737-808)?
There are two main systems of aircraft type designations: IATA and ICAO codes. The ones you mention are IATA codes (all 3-character) and are often used for shortened aircraft type designation in public information.
Both code types are listed here with the corresponding full aircraft types. As you can see, the IATA information covers the type and variant, including some features such as winglets, but not necessarily extended range models or sub-variants with different engines. It also differentiates between freighters and passenger versions.
Despite the additional digit, ICAO codes are less precise and often include several variants such as freighter as well as passenger variants of the same type.
The codes given in your question translate to:
(all passenger aircraft)
Unfortunately, as @Ralph J mentions, it's not always clear which type of code is used. Mostly, a 4-digit-code will be an ICAO code and a 3-character-code will use the IATA system, but because some ICAO codes only contain 3 digits, this isn't always reliable. For example, if you read "DC6", it could be the IATA code and refer to the passenger version, or it could be the ICAO code and you wouldn't know if it refers to the passenger or the freighter version.
Then there is the 737-700, which would, by that convention, translate to the "737". But that's the general type, the 737 that encompasses all of the variants. So you can foresee plenty of confusion if sometimes "737" means the Boeing 737, and other times "737" is short for the particular 737-700 variant.
So to avoid that confusion, they took the 7th letter of the alphabet, G, and used that instead of the "7" of "-700" and so now:
For the sake of consistency, they did the some thing with the 737-800, and in some places you'll see that referred to as the "73H" -- as the 737-700 is followed by the 737-800, the 73G is followed by the 73H. But in other contexts, you'll see the reference revert to the old convention, and
The link in @JulianHzg's answer specifies which codes are used in IATA and which in ICAO usage, although sometimes you'll see a code in isolation without necessarily knowing which system of reference it's coming from. In a pure, strict ICAO framework, the "B737" is ONLY the 737-700, with winglets, in a passenger configuration, but you'll see that used in places that aren't quite as strict in their conventions, and "73G" avoids confusion.
While the existing answers cover the most common uses of such codes (ICAO and IATA codes,) in order to further complicate things, there is actually a third possible meaning, as well. Airlines often assign a letter designator in order to specify a particular cabin configuration. For example, Delta has no less than 9 unique codes for its 757-200 cabin layouts, even though they're all exactly the same airframe. If you're looking at a website like SeatGuru (or the website of the airline itself,) these are the codes you'll see in reference to exactly which cabin configuration will be on the particular flight you're booking.
A given seat number may refer to a completely different seat or even not exist at all from one cabin configuration to another, even on the same type of airframe on the same airline, so these codes are used to disambiguate the configuration. Unfortunately, this also has the result of making the code itself even more ambiguous in the absence of context indicating which type of code it is.
While this type of code doesn't mean much to the pilot, it can mean a lot to the passengers. To go back to the Delta 757-200 example, on the 75A configuration, seat 2A is a normal First Class seat with 40" seat pitch. On the 75S configuration, on the other hand, it's a 76-inch-long flat-bed seat. Slight difference there. While the differences in economy usually aren't quite as dramatic, they can still be rather large. For instance, on the 75S configuration, seat 19F has a lot of extra leg room due to a missing seat in front because of an exit door, whereas it's just a normal economy seat on the 75A configuration.
As far as I know, this code is distinct from the customer codes discussed in this question, which would instead take the form of 757-232 for a 757-200 delivered to Delta Airlines. The code discussed in this answer denotes the current cabin configuration of the aircraft and takes the same form as the ICAO/IATA codes, whereas the code discussed in the other question denotes the customer who originally ordered the aircraft from the manufacturer. So, while all 757-200s initially delivered to Delta Airlines would be 757-232s, they can (and do) have many different cabin configurations, each with their own code of the form 75A, 75S, 75Z, etc.
