Code of planes how can a layman understand them 

The customer code in a Boeing model refers to the airline which originally purchased the jet.

For Example: The 777-247/ADV would mean 

777 -> Aircraft Model.

2 -> Model Series.

47-> Customer Code.

ADV -> Advanced or Extended Range or Cargo/Freighter.

 Airbus model numbers include a code for the jet's engine manufacturer. They are: 0 - GE; 1 - CFM; 2 - Pratt & Whitney; 3 - IAE; and 4 - Rolls-Royce. And an example could be,A340-313X 

A340 -> Aircraft Model.

3 -> Model Series.

1 -> Engine Manufacturer.

3 -> Model Subtype.

X -> Extended Range.

Germany - D-AIGU Lufthansa Airbus A340. So that would mean "GU" on the Lufthansa tail is the fleet number. Also note that in Germany many fleet numbers are simply the last two letters of the registration as in the Lufthansa example.

Now the detailed answer for the benefit of everybody  because Aviation  needs to be studied in DEPTH:

The rules for registration numbers of aircrafts in Germany have to be divided into 3 time frames:

Pre-1945 - Germany (Deutschland, Deutsches Reich, etc.)

1945 - 1990 - German Republics

1955 - 1990 - West Germany (Bundesrepublik Deutschland)

1952 - 1990 - East Germany (Deutsche Demokratische Republik, DDR)

1952 - 1956 (International Prefix DDR)

1956 - 1981 (International Prefix DM)

1981 - 1990 (International Prefix DDR)

1990 onwards - Germany (Bundesrepublik Deutschland)

This description covers time frames post-W.W.II (West) Germany  D-XXXX

1) D (international prefix: D)

2) - (dash, sometimes omitted, usually not)

3) character (0 to 9 or A - C, E - I, K - M, O)

4) character (0 to 9 or A - Z)

5) character (0 to 9 or A - Z)

6) character (0 to 9 or A - Z)

Min.: D-0001 & Max.: D-OZZZ

1) International Prefix: D (for Deutschland)

2) Separator: The dash (-) as Seperator of Prefix and Registration is usually displayed.

3-6) Registration: 4 characters, allowed are digits (0 - 9) and letters (A - Z).

The registration can be split into two main categories: numeric: 4 digits (D-0001 to D9999)

for  sailplanes/glider have registrations of this form;

the block 9500 to 9999 was reserved for some semi-military, government owned, aircraft during the 1960s;

letter: (4 letters D-xAAA - D-xZZZ), where x stands for one of the following aircraft type categories:

A - multi-engine, MTOW >20,000 kg,

B - multi-engine, MTOM >14,000 kg - 20,000 kg,

C - multi-engine, MTOW >5,700 kg - 14,000 kg,

E - single-engine, MTOW <=2,000 kg,

F - single-engine, MTOW >2,000 kg - 5,700 kg,

G - multi-engine, MTOW <=2,000 kg,

H - all rotory-wing, (Hubschrauber = helicopter)

I - multi-engine, MTOW >2,000 kg - 5,700 kg,

K - all powered gliders,

L - all airships, (Luftschiff = airship)

M - all ultra lights, (UL)

O - all manned free balloons,

the categories D, J, N, P - Z are not used (at this time).

Summary: D- plus 4 characters; either 4 digits (for sailplanes/glider) or 4 letters; if letter, the first letter depicts aircraft type category; some military/prototype aircraft were registered in the D-9500 to D-9999 series during the 1960s; no other rules exist, concerning: special purpose/prototype/evaluation,location.

I studied this aspect to get into the detail of how can TAM and Reliance Aircrafts have the same registration and am getting more deeper into the same.