All praise is due to Allaah alone, and peace and blessings be upon the Messenger, his family and his Companions.
"As-Salafiyyah" is an attachment or affiliation to the 'Salaf'. And "The Salaf" are the Companions of the Messenger of Allaah (salallaahu 'alaihi wassallam) and the Imaams of guidance (may Allaah be pleased with them) from the first three generations, those whose goodness Allaah's Messenger (salallaahu 'alaihi wasallam) bore witness to in his statement:
"The best of mankind is my generation, then those who follow them, then those who follow them. Then there will come people where bearing witness of one of them will precede his taking an oath and his taking an oath will be his bearing witness."
[Reported by Imaam Ahmad ibn Hanbal in the 'Musnad', 'Al-Bukhaaree', and Muslim]*
And "The Salafiyyoon [or Salafees]" is the plural of "Salafi", an attachment or affiliation to the Salaf, and its meaning has already preceded above - and they are those who traverse upon the Methodology (Manhaj) of the Salaf in following the Book of Allaah and the Sunnah [of the Messenger of Allaah], and inviting to them, and acting upon them, and they are, due to this, Ahlus-Sunnah wal-Jamaa'ah.
* Imaam Ahmad: (4/426). Al-Bukhaaree: (2651). Muslim: (2535).