I am reading Unconventional Success written by money manager David Swensen and one of the important concepts he talks about is the idea of core assets, which he defines as:

Core asset classes share a number of critical characteristics. First, core asset classes contribute basic, valuable, differentiable characteristics to an investment portfolio. Second, core holdings rely fundamentally on market-generated returns, not on active management of portfolios. Third, core asset classes derive from broad, deep, investable markets.

Core asset classes discussed in the book include domestic (US) equity, US treasury bonds, inflation-linked bonds, foreign developed equity, emerging markets equity and real estate. Domestic corporate bonds, high-yield bonds, tax-exempt bonds, asset-backed securities, foreign bonds, hedge funds, leveraged buyouts and venture capital fall into non-core category. The bottom line is that average investors can focus their investing on core assets and safely ignore everything under the non-core category.