Main features:

automatic memory management;
advanced exception handling capabilities;
a rich set of input/output filtering tools;
a set of standard collections, such as an array, a list, a stack, etc.;
the presence of simple tools for creating network applications (including using the RMI protocol);
the presence of classes that allow you to make HTTP requests and process responses;
built-in language tools for creating multithreaded applications;
unified access to databases:
at the level of individual SQL queries - based on JDBC, SQLJ;
at the level of the concept of objects that have the ability to be stored in a database - based on Java Data Objects (English) and Java Persistence API (English);
template support (since version 1.5);
parallel execution of programs.

What is the difference between JRE and JDK:

JRE briefly - for work. Java Runtime Environment (JRE) is the minimal implementation of a virtual machine required to execute Java applications, without a compiler and other development tools. It consists of a virtual machine - Java Virtual Machine and a library of Java classes.
JDK in short - for programming. Java Development Kit (abbreviated JDK) is a Java application development kit distributed for free by Oracle Corporation (formerly Sun Microsystems), which includes the Java compiler (javac), standard Java class libraries, examples, documentation, various utilities and the Java Executive System (JRE)