These are very general questions that require complex responses to address with any real accuracy. Several notable primers, which I will provide for you at the end of this post, can give you more complete answers, but I will cover the basics here in hopes that it holds you over until you can follow it up with further reading.
In short, the Jacobites were dynastic adherents to the Stuart line of monarchs in the three kingdoms of Britain and Ireland. They effectively existed from the moment of the 'Glorious Revolution' in 1688 roughly to the time of the death of Charles Edward Stuart (Bonnie Prince Charlie) in 1788. Some historians refer to this as the Jacobite Century or the Jacobite era, and the term comes from the Latin for 'James' (Jacobus), the progenitor of this phase of the Stuart dynasty.
There are many differing arguments for what Jacobitism represented through its hundred years of existence, and different scholars will focus on different qualities. My own work characterizes a common thread of Jacobite ideology through its entire life with a sort of diffusion of mutable and even appropriated 'causes' as it moves into its later era. In its most basic form, Jacobites wanted to see a return of the Stuarts to the British throne, to which they were considered to be rightfully entitled and in fact ordained by God. After the Treaty and Act of Union between England and Scotland in 1707, Jacobitism carried a powerful current of anti-unionism in Scotland that shared some qualities with but was not equivalent to modern notions of independence. As time progressed from the Union and as many Britons on both sides of the border became more comfortable with the economic benefits of that parliamentary marriage, Jacobitism starts to slowly transform into a more generalized movement of opposition to the Hanoverian government of George II and the established Presbyterian Church of Scotland. Therefore, to really understand the parameters of Jacobite ideology, it must be considered from many angles and properly placed into context.
A series of military risings (or rebellions) represented the most effective form that Jacobitism would take during its lifetime, with the effort in 1715-6 being the largest, and the final one in 1745-6 arguably the closest to succeeding. Though these eighteenth-century actions began and ended in Scotland, and though that nation also provided the majority of the army's constituents in these martial affairs, Jacobitism was always an international movement inextricably linked with Continental power politics. Along with both Highland and Lowland Scots, on the battlefields were regular Spanish troops (1719) and Irish regiments in French Service (1745), with Northern England teasing a Catholic and Anglican population base that covertly supported Stuart claims but, excepting meager contributions, largely stayed out of the actual fighting.
Characterizing Jacobite (or any other) effectiveness as 'good' or 'bad' is not something that is really done in scholarly historical study. Instead, we can examine motive and aptitude and test it against objectives and legacy. Again, fully addressing these qualities could easily fill a book by itself, but we can note a few of the most salient points in hopes that they collectively go some way toward providing a solid base from which to expand further as your interest dictates.
The fact that Jacobitism survived for a hundred years during an incredibly transformative period of European history is significant, as was the threat it brought to bear upon the British state during both the Williamite and Georgian administrations. This is demonstrated by examining British reactions in both attitude and policy generated to quell Jacobite intrigue and activity. Though typical Whig historiography delights in marginalizing the Jacobite threat and extolling the progressive virtues of Hanoverian polity, all it takes is a read through any of the State papers of the era to see how dire the perils of rebellion and treason at home were really regarded – especially while Britain was deeply engaged on the Continent during the War of the Austrian Succession. The formative years of British imperialism were birthed in Scotland after the Battle of Culloden in April 1746 and exemplary 'civilizing' was abhorrently inflicted upon recalcitrant Scottish communities with extreme prejudice, paving the way for imperial policy in the Far East, the Caribbean, and the North American colonies that would themselves rebel just thirty years later.
The ultimate failure of Jacobitism can be predicated on a number of factors, including incompetent military leadership and inadequate support from established sympathetic nations like France and Spain. Some scholars question the potency of covert political Jacobitism when not expressed in its martial form, and others consider it to have always been a doomed effort due to its lack of popularity and impulse to overreach. On this question of popularity, what stands out is that Jacobitism in its last years became decentralized under entitled and disparate leadership that conveyed convoluted and sometimes contradictory messages to its would-be supporters. We must therefore question the ability of Jacobite ideologies to widely appeal to the British population and whether that is any indication of a reluctance in 'modern' Britons to accept more conservative, traditional, Divine-Right rule from a dynasty of exiled kings.
Below are a few excellent introductory books that can be considered essential reads about Jacobites and Jacobitism, explored from different angles. I hope you find them valuable and that the above post helps with your questions.
With best wishes,
Dr Darren S. Layne
Creator and Curator
The Jacobite Database of 1745
Research Blog: Little Rebellions