Formula fields can either be

Expressions as in ${entity.budget * 0.10} where entity is the project OR
contain a mix of expression and text as in The Project ${entity.name} was created on ${entity.created}.
This will print the output as : The Project Sample Project - Office Setup was created on Thu Jun 20 08:30:16 EDT 2013.
contain a mix of expression and text as in <a href="http://mywikiserver/project/${entity.id}>Wiki Link</a>.
This will print 'Wiki Link' as the output which is hyperlinked with the link substituted with the entity's ID.

When formulas are evaluated, anything enclosed in ${..}is treated as an expression and evaluated. Everything outside it is copied to the output verbatim as is.

${entity.budget} will display the budget of the entity.
{entity.name} will be printed as {entity.name} in your report or the details page.

You can also use conditional logic in the formula.

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