alatka

1. tool; instrument
   Za ovu popravku treba mi posebna alatka. — I need a special tool for this repair.
   Ova aplikacija je odlična alatka za učenje jezika. — This app is a great tool for learning languages.
2. slang vulg. penis; tool
   Stalno se hvali svojom alatkom. — He's always bragging about his tool.

Alatka is a single tool, while alat usually means tools collectively.