GIVT is General Invalid Traffic and SIVT stands for Sophisticated Invalid Traffic. The MRC (Media Ratings Council) has devised a definition for it. It avoids the use of the fraud in blanket terms and hence gives an "invalid" traffic classification.
GIVT as per MRC consists of traffic identified through routine means of filtration executed through application of lists or with other standardized parameter checks. Few examples of GIVT traffic include "Data Center Traffic", "Bots, Spiders and Crawlers", "Activity based filtration", "Non browser UA Headers or other unknown browsers", "pre-fetch or browser pre-rendered traffic".
SIVT as per MRC consists of more difficult to detect situations that require advanced analytics, multi point corroboration / coordination, significant human intervention, etc to analyse and identify. Few examples are "Differentiating human and IVT if they are originating from the same source", "Bots masquerading as legit users", "Hijacked user sessions, devices etc", "Non-viewable / obfuscated ad serving", "Invalid Proxy Traffic", "Adware and Malware", "Incentivized traffic", "Falsified viewable impressions", "Falsely represented sites", "Cookie stuffing, harvesting and recycling", "Falsification of location data".
You can also download the MRC Invalid traffic guidelines from here