What is selenium and cucumber difference?

Selenium is an open source tool for automating browser-based Web applications which is Application under Test(AUT) this tool to do automated testing.
We can take out the selenium test script in order to perform Regression Testing of applications across any OS and any browser.
Selenium is a suite of tools which is automate web browsers in different platforms for simple functional testing.
And it supports many languages, runs in different operating systems but only for web applications to automate.
Selenium suite is a combination of four tools, But to start with selenium tool you require knowledge on programming languages.

Cucumber is a tool that based on Behavior Driven Development (BDD) framework which is used to write acceptance tests for web application  is extension of Test Driven Development.
It allows automation of functional validation in easily readable and understandable format (like plain English) to Business Analysts, Developers, Testers, etc.
Gherkin is the language that Cucumber uses to define test cases in plain meaningful English text using a grammar.
It is written in Ruby but it can be used to test Java, c# and python written codes.
It is focuses on User Acceptance testing this is the main feature of Cucumber.

Differences between Selenium and Cucumber:
What is Selenium?

Selenium is a browser automation testing tool which is capable of interacting with the HTML elements With selenium webdriver of a web application to simulate a user activity and cross browsers testing.

Selenium

Selenium is a testing framework.
The selenium process makes the testing more reliable and dependable.
In Selenium test scripts are written based on test cases and requirements.
In Selenium, syntax errors are easy to find while developing.
Plug-ins in Selenium works slower as compared to Cucumber.
We can write scripts in a number of programming languages using Selenium WebDriver and can be a great asset for multiple OS and cross browser testing.
require to have knowledge in programming language.
What is Cucumber?

Cucumber was created to drive Behavior Driven Development (BDD) process, such that the customer can describe their requirements as a series of examples called scenarios, in plain text files using the gherkin language in the Given When Then format.

Cucumber
Cucumber is a behavior driven development tool having feature file.
Cucumber testing is not reliable comparatively.
In Cucumber scenarios are written and verified by Business Analyst before execution.
In Cucumber syntax errors are not getting into notice while writing, it will define the application behavior.
This is kind of plug in is not independent automation tool, business analyst friendly tool.

Hope this Helps :)