The BBC does a pretty good job in explaining the current disagreements that are arising in current EU budget discussions:

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EU Rebate: Background

All EU countries contribute to the EU budget, and in return benefit from EU spending in their countries.

Because the bulk of the EU budget is spent on supporting farmers’ incomes, countries with a large agricultural sector (like Spain, Portugal and Greece) generally get more back than they put in.

Large countries like Britain and Germany are net contributors to the EU budget, while Italy and France are broadly neutral.

However, in 1984 Britain negotiated a reduction of two-thirds in its net contribution, to be paid by other EU members, on the grounds that as a relatively poor member state it paid too much and received relatively little in return.

Now the other EU members, including the 10 new members mainly drawn from former communist states in Eastern Europe, want to abolish or reduce that rebate, on the grounds that Britain is no longer one of the poorer member states, and support for agriculture is a diminishing part of the EU budget (more…)