In computer programming terms, an algorithm is a set of welldefined instructions to solve a particular problem. It takes a set of input(s) and produces the desired output. For example,
An algorithm to add two numbers:

Take two number inputs

Add numbers using the
+
operator 
Display the result
Qualities of a Good Algorithm
 Input and output should be defined precisely.
 Each step in the algorithm should be clear and unambiguous.
 Algorithms should be most effective among many different ways to solve a problem.
 An algorithm shouldn't include computer code. Instead, the algorithm should be written in such a way that it can be used in different programming languages.
Algorithm Examples
Algorithm to find the largest among three numbers
Algorithm to find all the roots of the quadratic equation
Algorithm to find the factorial
Algorithm to check prime number
Algorithm 1: Add two numbers entered by the user
Step 1: Start Step 2: Declare variables num1, num2 and sum. Step 3: Read values num1 and num2. Step 4: Add num1 and num2 and assign the result to sum. sum←num1+num2 Step 5: Display sum Step 6: Stop
Algorithm 2: Find the largest number among three numbers
Step 1: Start Step 2: Declare variables a,b and c. Step 3: Read variables a,b and c. Step 4: If a > b If a > c Display a is the largest number. Else Display c is the largest number. Else If b > c Display b is the largest number. Else Display c is the greatest number. Step 5: Stop
Algorithm 3: Find Roots of a Quadratic Equation ax^{2} + bx + c = 0
Step 1: Start Step 2: Declare variables a, b, c, D, x1, x2, rp and ip; Step 3: Calculate discriminant D ← b^{2}4ac Step 4: If D ≥ 0 r1 ← (b+√D)/2a r2 ← (b√D)/2a Display r1 and r2 as roots. Else Calculate real part and imaginary part rp ← b/2a ip ← √(D)/2a Display rp+j(ip) and rpj(ip) as roots Step 5: Stop
Algorithm 4: Find the factorial of a number
Step 1: Start Step 2: Declare variables n, factorial and i. Step 3: Initialize variables factorial ← 1 i ← 1 Step 4: Read value of n Step 5: Repeat the steps until i = n 5.1: factorial ← factorial*i 5.2: i ← i+1 Step 6: Display factorial Step 7: Stop
Algorithm 5: Check whether a number is prime or not
Step 1: Start Step 2: Declare variables n, i, flag. Step 3: Initialize variables flag ← 1 i ← 2 Step 4: Read n from the user. Step 5: Repeat the steps until i=(n/2) 5.1 If remainder of n÷i equals 0 flag ← 0 Go to step 6 5.2 i ← i+1 Step 6: If flag = 0 Display n is not prime else Display n is prime Step 7: Stop
Algorithm 6: Find the Fibonacci series till the term less than 1000
Step 1: Start Step 2: Declare variables first_term,second_term and temp. Step 3: Initialize variables first_term ← 0 second_term ← 1 Step 4: Display first_term and second_term Step 5: Repeat the steps until second_term ≤ 1000 5.1: temp ← second_term 5.2: second_term ← second_term + first_term 5.3: first_term ← temp 5.4: Display second_term Step 6: Stop