Understand Index Funds for Complete Beginners

You have been told index funds are sensible but nobody explained what they are or how you would even buy one. This starts from first principles: what an index fund is, how it differs from picking shares, why it is the default recommendation, and the practical steps to buy one. It speaks to whatever is actually worrying you about starting.

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You have heard that index funds are a sensible way to invest but do not understand what they are, how they work, or how to buy one. You have [never invested / only have a pension / a small amount ready to invest]. Your main concern is [losing money / picking the wrong fund / not knowing where to start].
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<task>
**Explain index funds from first principles:**
1. What an index fund is and how it differs from picking individual stocks or using an active fund manager
2. Why low-cost index funds consistently outperform most actively managed funds over the long term
3. What 'diversification' actually means in practice and why it reduces risk
4. How to buy an index fund in the UK: platform, wrapper (ISA/SIPP), and fund selection
5. The one mistake beginners make that costs more than picking the wrong fund
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<output_format>
- Index fund explanation (plain language, one paragraph)
- Active vs passive comparison (short table: Factor | Active fund | Index fund)
- Diversification explained with a concrete example
- Step-by-step buying guide (UK-specific, 5 steps)
- The biggest beginner mistake (one paragraph)
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