The most basic of entry methods. Numbers are calculated as they are entered resulting in 2 + 3 × 4 = 20.
Algebraic with precedence
Same as above, but precedence rules are applied (multiplication comes before division etc) resulting in 2 + 3 × 4 = 14 as is it calculated like 2 + (3 × 4).
Calculations are entered as expressions which can include keywords like SIN, COS, LN, etc.
Calculations are entered as complete formulas and executed in one go. Closely resembles BASIC expressions.
Intel 80x86 machine language
A program written in assembly language consists of a series of instructions--mnemonics that correspond to a stream of executable instructions, when translated by an assembler, that can be loaded into memory and executed.
Reverse Polish Notation
Reverse Polish notation (or just RPN) by analogy with the related Polish notation, a prefix notation introduced in 1920 by the Polish mathematician Jan Łukasiewicz, is a mathematical notation wherein every operator follows all of its operands. It is also known as Postfix notation and is parenthesis-free.
In Reverse Polish notation the operators follow their operands; for instance, to add three and four, one would write "3 4 +" rather than "3 + 4". If there are multiple operations, the operator is given immediately after its second operand; so the expression written "3 − 4 + 5" in conventional infix notation would be written "3 4 − 5 +" in RPN: first subtract 4 from 3, then add 5 to that. An advantage of RPN is that it obviates the need for parentheses that are required by infix. While "3 − 4 × 5" can also be written "3 − (4 × 5)", that means something quite different from "(3 − 4) × 5". In postfix, the former would be written "3 4 5 × −", which unambiguously means "3 (4 5 ×) −".
Interpreters of Reverse Polish notation are often stack-based; that is, operands are pushed onto a stack, and when an operation is performed, its operands are popped from a stack and its result pushed back on. Stacks, and therefore RPN, have the advantage of being easy to implement and very fast.
Note that, despite the name, reverse Polish notation is not exactly the reverse of Polish notation, as the operands of non-commutative operations are still written in the conventional order (e.g. "/ 6 3" in Polish notation corresponds to "6 3 /" in reverse Polish, both evaluating to 2, whereas "3 6 /" would evaluate to 0.5). Numbers are also written with the digits in the conventional order.
Calculations are entered and executed in a spreadsheet.
Quotes taken from Wikipedia