There are three broad models for a deal, which is looking ever more likely.
A full coalition: A clutch of cabinet posts for the Lib Dems. Collective responsibility. A multi-year legislative agenda and a complex arrangements for co-operating in government, resolving disputes, sharing information, developing policy. Cameron wins stable backing for a full term, take bulk of credit while sharing fallout from unpopular decisions. Both leaders face possible uprising from activists. Lib Dems struggle to maintain distinctive identity. The least likely option.
Enhanced Confidence and Supply: Ministerial jobs for a few Lib Dems. But collective responsibility limited to agreed programme on those portfolios. Package of Lib Dem policy in Queen’s speech. Agreement on broad terms of fiscal consolidation and first Budget. Option to “agree to disagree” on policy, if required. Future legislation on case by case basis. Currently preferred model for politicians in New Zealand and other parliamentary systems. Gives Cameron a degree of stability. In practice the public see Tories as “the government”. Clegg wins power, resources and a chance to maintain Lib Dem brand. But smaller party typically loses vote share in next election.
Minimal Confidence and Supply: Concessions to Lib Dems on some policy in first Queen’s Speech. Opportunity to put some Clegg tweaks to first Budget. Agreement on broad path of fiscal consolidation. But largely a Tory programme. All other legislation taken on case by case basis. Most unstable of all the options. Permits the Lib Dems to remain as an opposition party with proven power to influence. Arms length from Tory cuts.
For more, I’d highly recommend this Institute for Government report.