What is the most efficient way to store tags in a database?

One item is going to have many tags. And one tag will belong to many items. This implies to me that you'll quite possibly need an intermediary table to overcome the many-to-many obstacle.

Something like:

Table: Items
Columns: Item_ID, Item_Title, Content

Table: Tags
Columns: Tag_ID, Tag_Title

Table: Items_Tags
Columns: Item_ID, Tag_ID

It might be that your web app is very very popular and need de-normalizing down the road, but it's pointless muddying the waters too early.

Actually I believe de-normalising the tags table might be a better way forward, depending on scale.

This way, the tags table simply has tagid, itemid, tagname.

You'll get duplicate tagnames, but it makes adding/removing/editing tags for specific items MUCH more simple. You don't have to create a new tag, remove the allocation of the old one and re-allocate a new one, you just edit the tagname.

For displaying a list of tags, you simply use DISTINCT or GROUP BY, and of course you can count how many times a tag is used easily, too.

If you don't mind using a bit of non-standard stuff, Postgres version 9.4 and up has an option of storing a record of type JSON text array.

Your schema would be:

Table: Items
Columns: Item_ID:int, Title:text, Content:text

Table: Tags
Columns: Item_ID:int, Tag_Title:text[]

For more info, see this excellent post by Josh Berkus: http://www.databasesoup.com/2015/01/tag-all-things.html

There are more various options compared thoroughly for performance and the one suggested above is the best overall.