SAP tcodes (transaction codes) are an easy way of executing SAP programs and reports. These programs are written using ABAP code and essentially make up an SAP system interface and functionality. I.e. almost every time you perform a specific action within SAP, such as executing a tcode, clicking on a link, running a report or even creating a report you are executing an SAP program.
As well as the programs that make up the SAP development environment there are thousands of programs/tcodes which have already been created by SAP which perform a multitude of very useful functionality. Also see SAP ABAP reports which are also written using ABAP but offers the user a large amount of reporting functionality.
Popular SAP Transaction Codes
SAP object navigator
SAP directory structure
BAPI Explorer to view SAP bapis
table maintenance for SAP databases
Batch input processing
Maintain Output Determination
IDoc and EDI