SRS = Spaced Repetition Software
An SRS is essentially a modern version of paper flashcards, and its purpose is to make things, such as foreign languages, easier to remember.
The idea behind an SRS is as follows:
1.) Enter whatever information you want to remember into an electronic flashcard.
2.) Review the cards for a few minutes and grade yourself on how well you remembered the information.
3.) Repeat for a few minutes every day.
4.) Remember the information forever because of its optimized algorithm designed for efficient review.
As you can see, the an SRS is similar to old-fashioned flashcards. The big difference, though, is that the SRS contains an algorithm that is optimized making your reviewing as efficient as possible for maximum retention. In addition, an SRS is electronic, which means it's easier to review your cards from anywhere (your computer, someone else's computer, mobile phone).
I recommend using Anki as your SRS
There are several to choose from, but I believe the best is Anki.
Below are some reviews of SRS's in general and also Anki specifically.
"What is an SRS?"
"Spaced Repetition: Never Forget Vocabulary Again"
"How I learn new vocabulary with parallel texts"
"How to Learn a Language on Your Own"
"2000 Kanji in three months"
"Use It or Lost It - from Anki User Manual"