I need to download some cities boundaries polygons and store them in a Postgis database. Is this possible? Using what tools?
Over at geofabrik you can download up-to-date shapefiles with selected osm attributes. They have even have categorized per country.
Once downloaded and sorted the data that you need; you can upload your data to your postgis using a variety of tools depending on your project: Qgis, gdal, shp2pgsql and others.
Also take heed, the data are in WSG84 (4326) so you might need to reproject them to your local Coordinate System while or before upload them to your database.